Is the actual key sticking physically, like encountering friction in the up-down action? Or are the keys 'virtually' sticking where the actuation itself is not reliable?
Sometimes reseating the keys (pop them out so you can see the spring, tilt the keyboard far back so it is upright facing away from you, then re-install the keys) will correct these things.
If there is a legitimate dirt problem inside causing erratic actuation and it is not possible to physically extract the offending dirt, the only option that would carry any longevity is referred to as a "
bolt mod" (click for link). This becomes necessary at one point or another anyway on all Model M-type keyboards due to the cost-reduced plastic rivet design - contrary to popular belief the Model M design, which refers to the switching method not the keyboard, is actually quite fragile. The act of doing the bolt mod is not what fixes it; instead you use the opportunity while it is opened up to gently but thoroughly clean all surfaces you can't normally access.
Everyone I know of who has done this successfully is in agreement that it is an upgrade in feel at the same time.
I believe there are currently people performing this process for a fee for those who are uncomfortable doing it themselves...