Strategies for Long-Term Residential Rentals

I've got a great house with really incredible tenants.  They pay early, treat the house as though they own it, etc.  They mentioned about wanting to re-up their currently lease by signing a long-term lease.  I think this is a great idea, as it doesn't force me to have to look tenants again, but I'm worried about how do I ensure that the rent I'm charging stays with market rent.  Does anyone have any experience with this?  The only thing I can think of is writing in a rent bump of $X/year, but I don't want to short myself in the long run.