Best way to go about raising rent?

[Lucas Hammer]

So I have a two flat that I've owned a little over a year (first property) and we inherited a good tenant and immediately re-signed a lease with him that runs October through September, so it's ending toward the end of next month. His rent is definitely under market, so we have to raise it due to rising expenses, but we don't want to raise it enough for him to leave (unless he's planning on it anyway; we want to have our game plan in place before discussing). Do you guys typically go by percentages or just keep it around market average based on the property? Also, how much notice do you give before raising rent? I know you have to give two months notice if they have to be out, but I couldn't find a specific law about rent increases. Thanks!