A lot of people seem to place a limit on the number of troops you can raise, matching it to province levels or province levels times a multiplier (like 1.5). Knights are often limited to 1 unit per 4th level province, with more at 7th and 9th levels. That's a fair method of control, I think. These are things I've seen before in multiple PBEMs--Ericthecleric used them in his last one.

My own preferred method may not make much sense excerpted from my somewhat wider house rules. I'll present it in another thread, because I would like feedback.