Hi Scott,
Here's my two cents worth. Maybe in the end, you'll find that's all it's worth!
Could you offer additional savings on a certain manufacturer every month (with some exceptions, of course), or a particular road name, or perhaps a particular type of train or rolling stock, etc. It wouldn't have to be much either. Maybe a few bucks off on shipping or some other idea that won't end up costing you a lot of money.
Since I have an automotive background, how about accepting pre-approved trade-ins from your customers for something they'd rather have? As you know, many people phase out of certain items for a variety of reasons, so they're looking to liquidate some things. Many people either can't, or won't sell at shows or on-line, so this would be a way for you to obtain new inventory and for the customer to trade for something he wants while thinning down their current collection.
To increase your email list of customers to send new product availability to, perhaps you could hold a drawing for all new contacts you receive within a certain time period to let's say a gift certificate or anything else you deem appropriate.
Finally, since you advertise in many forms of different media, you could offer a special tracking code for a small credit off a customer's minimum order. This is also a good way to track where your advertising dollars are being used most effectively. For ads that don't generate a pre-determined amount of traffic, you can drop those ads and spend that money in current or new advertising.
Maybe you're using some of these ideas now, or maybe you tried them earlier only to find they don't really work for you, but I've found these low-cost methods helped my business grow. Best wishes for continued success. Thanks for all you do for the hobby.