I usually buy a little something every time I visit the LBS. Another patch kit. Maybe a doodad I'd planned to buy on Amazon or Nashbar, but pay a couple bucks more at the LBS for the same time -- brake pads, mirror, whatever I may need, as long as the price is reasonably within range of the online vendors. Usually the LBS prices are either identical to online vendors, or only a couple of bucks higher.

And I gladly pay them for their expertise in maintenance and repairs that I don't feel qualified to do, or for which I don't have the tools. Recently I paid ten bucks to straighten a wobbly rim. Took less than 10 minutes, and worth every penny. It'll help me delay buying a new wheel awhile longer. And I didn't have the expertise to fix that wobble as effectively as they did in a few minutes.

I've done that for years with local camera shops too. If I visit, and especially if they let me try some demo gear, I'll buy a little something before leaving. These are usually affordable but profitable items for the shop.