Our best sellers have been the Quest Bars and the Nuts N More peanut and almond butters. We typically have a flavor of the week in both that we discount a few dollars that really helps to boost the sale for the product. Although we have a specific area for product our specials are always at the front counter, and are usually a last minute pick up item.
Your best seller will generally be the one that the person behind the counter likes most. They will buy it and tell others about it.
Funny that the staff will eat just about as much as the members. Unless the staff are stealing the bars.
Samples are good at the counter as well. Talk to your distributor to see if they can give you some. Also ask the distributor if you could get a deal on buy a greater bulk for cheaper.
Known bars usually sell better as well.
Also you need to believe in the product you are selling as well. Plan on skipping a few meals and eating the bars instead. Eat them in front of everyone!!
We carry some CDs with good music for meditation/stress reduction/yoga. The thing is really good ones are hard to get via the normal routes and need some knowledge to pick out. Given the popularity of mindfulness/stress reduction as a balance to the hard core workout perhaps some things like that might work (or the nice flax seed eye bags, things like that).
Have you polled your members to see if there is something they would like to see you carry?