1. Increase your pricing (if you have room to move) just don't get out of control and set them too high. Also leave current customers at their current pricing ideally.
2. Advertise more...whether it's Good Adwords, PPC, etc...
3. Do some SEO improvements, that can do wonders and I guess falls under advertising in a way...marketing.
4. Diversify yourself from other resellers, why do I want to choose you over "BobsReallyGoodHosting" (That was a fake example...)
5. Find a way to drive social media offerings...this also falls into advertising in a way.
Reselling will only grow as much as you put the time into it. Put only little effort/time into Reselling, well you'll be lucky to grow just little amounts at a time. Do the leg work and really work to make it grow and you'll "typically" see better results.
Build relationships with your clients, especially ones that will in turn also become resellers of yourself.