Though I was told it was not necessary my Fastrack is screwed down. Many of the trees are puttied in place. Many trees are free standing as well. The towers and other tall structures are screwed or nailed in placed.
Pretty much everything else is held in place by gravity.
I do have minor problems of things being knocked over when reaching to far places.
Is this pretty typical of your layout.
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Gravity is probably the biggest holder. My trees, telephone poles, Burma Shave signs, switch number signs and all of my figures are glued in place. I also knock a few things over when reaching, mostly trees.
My layout is still a temporary affair, and so apart from a few screws to hold the track in place, nothing at all is secured. 
I use putty for trees and people. Operating accessories are screwed down, as is the track. Automobiles and non-operating structures are left to the mercy of Sir Isaac's force, which may or may not involve the use of gravitons.
I am still building and I am always changing something. Drawbridge and track on other side only things screwed down at this point. Hills, mountains and tunnels are tape over woven cardboard but, once I get some plaster cloth hardened in shape I shold be able to detach the base from the benchwork.
Probably the only things not secured are buildings and model cars. I use Squadron green putty(have Life Like green mats) on the base of my trees and either screw down or thumb tack my telephone poles in place. Bridges are secured and figures of people as well.
Lee Fritz
Everything. Cars, buildings, people, and Super Streets track, but that might change with a bigger layout in the future. But I only run a couple of trolleys, not trains at this point.
Buildings and cars are not secured. Two of my accessories, the Lionel sawmill and operating freight station are fastened to the layout using screws as I have found that to be an good way to adjust the amount of tension / vibration needed to make the accessory operate properly.
Curt
Everything but the track, control panels for the remote switches and the whistle is not fastened down. Even the roads are removable. This makes the small layout easier to move if the need arises or if I want to renovate the layout. Everything that is fastened down is held on with screws so even those things can be removed easily if need be.
On my 8’ x 5’ display, with 8 loops all the track is screwed down to the roadbed and one billboard is secured to the vertical facing of a bridge. Though I don’t have many accessories, everything else floats in place including; 7 individual tiers, two mountain sides, 6 bridges, 12 tunnel portals, 2 beacon towers and two additional billboards. The display sits a top bench work that rides on 14 ball casters and has been moved on numerous occasions while all the trains are operating without any incident (not even to the beacon lenses).
Track and roadbed is fastened by small track screws and tacks. Ballast is glued down using Matte Medium as is the ground cover. Lichen is glued using white glue. Anything I want to be removable without damage I do not use any type of glue. Trees, animals, figures, and other small items I use Quick Stick Holding Wax. Buildings are just sitting in place.
Gandy
Curt nailed it for me, buildings (mostly D56) and cars are not secured.
Figures are pinned down with wire. Vehicles, for the most part, are held down by gravity. We did have to wire a couple down because someone bagged our steam roller from the "Steam Roller Incident" scene. Needless to say, we wired down the ambulance from the "Mac Attack" scene.
Buildings are held down by gravity except where they are part of an access hatch cover.
Everything except my buildings and mountains are secured to the platform. Keep in mind that my entire platform top is covered in cork. All the pole lights are affixed with small brads or screws into the cork. My roadways that are made from rubberized underlayment are affixed with thumbtacks whose heads are painted in the same color. Track is screwed in strategic places into the cork with a dab of clear latex caulk between the screw head and track ties. I use LeMax sticky wax for small figures and signs and accessories which is removable. I use this on tree bases as well. Metal accessories such as operating gate man are screwed into the platform but have a rubber faucet washer between the screw head and metal platform to avoid vibration. I also use cell foam insulation pieces underneath the bases (the stuff you can buy in a roll that is normally used to seal tops of concrete foundations before installing a sill plate). Surface wiring is contained through the use of small U-shaped staples that easily press into the cork. Small nails into the cork affix power line polls with the heads painted with a dab of appropriate color to mask them. Bridges are definitely screwed down I have found that vibrations not only create a noisy layout but also cause the scenery and accessories to creep and turn. That is why it is imperative for me to fix them down. The advantage with the cork is that it heals when screws and nails are removed. This is especially useful when I want to change or modify something. For ballast, I use double-sided tape along the track, pour the kitty litter on it and then wet the top with a 50/50 mix of glue and water--this is removable. Another option I use in spots is aquarium stones with a bit of glue/water mix on top. Stones inside the rails are not glued but held in place by the rails, ties, and side ballast. In certain portions of the layout I cut pieces of green felt and affix it to the cork with thumbtacks to simulate green grass or use the commercial grass paper with thumbtacks. This prevents curling.
Almost everything on my 5x8 layout is held by gravity, including the FasTrack. Some of the figures and trees are held by Mini-Hold Wax.
Currently, nothing on my (unfinished) layout is secured. Even the landforms (mountains) trestles, grass-mats and track are not yet secured. I figure I will glue everything down at some point. But glad you asked the question, I am interested in what others are doing.
Anything on my layout that is secured is done with velcro.
im set up pretty much like you are michael.my light towers and a few operating accessories are free standing.
thop.........
Right now, just about everything is held down by gravity. On the Ceiling Central RR, I still have less than a quarter of the shelving to hang. I just have cork sheets and with track on them - nothing secured. I still want some wiggle room to get everything to fit in space. when you only have about 9 inches from the top of the doors to the ceiling, there isn't much room to get my tri-focal enhanced eyes in there to see what I'm doing. Thanks to the weight of O Gauge they all run fine. I have some buildings, vehicles, and poles up there, just held with gravity.
On the Patio Pacific RR, I have a loop of O42 RealTrax fitted together with their clips on the underside. I can move it in and out in one piece easily. The same goes for my Carpet Central RR but it is a loop of O31.
Once I acquire a spare room in the house, I will mount the track permanently. The same goes for most trees and poles, except ones that get in the way when I am working on something. The buildings will be removable for lighting and repair.
Buildings, Oil Pumps, cars, trucks and a corral stay in place by gravity. I have screwed my Fastrack down because of some shift from time to time creating problems like the engines losing legacy contact, don't have a clue if that was what causing the signal problem but screwing the track down solved the problem.
Brent
All five loops secured by screws every three feet. Everything else is free floating. Other than the buildings, I like being able to move signs and trees from time to time.
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