If you stand, be sure that you still have one more gear "in the hole." If you need to sit again, you'll probably want an easier gear.

Here is something that really helps me: I don't "fight the hill." I mentally "work with the hill". So, I hit the hills as fast as I can so I have some momentum. I'll sit until my cadence slows to ~60, depending on how my legs feel that day. Usually that gets me over the hill - or close to it.