Blow on the reed without putting it in the oboe. Then see how fast you can make the vibrations. Hold the reed firmly to your bottom lip--try making sounds at different angles. I know it sounds crazy, but it really does work.
i used to face the same prob..my teacher always said, 'relax more, relax more'..so i always practice with a relax embouchure..and now, i build my tone beautifully n my lips become stronger..dont know how to say, just relax while practice n bear in mind 'be more relax then what u can think'..i know it works!
My teacher has had me do this reed-alone exercise.
I put the reed in my mouth like I'm about to play without holding it with my hand.
I make the reed chirp and see if it will move or stay put. If my embouchure is strong, then it will stay put. If it's not, it will wiggle around and not make much sound.
if you're practicing and gradually taking longer practice times, that's the right thing to do.
1) Eat something before you practice. Not a whole lot so that you are so full that it's hard to breathe, but a light meal or snack. It really helps stave exhaustion if practicing at the end of the day.
2) Double layer (un-gummed!) cigarette paper or perm papers and layer over top and bottom teeth. The paper will take a little getting used to, but this will allow you to practice for very long periods of time as your teeth will not make direct contact with your skin. There is no other way you can practice for 2+ hours. You should have an easier time playing without the paper at rehearsals since you'll have built up the endurance for it.
(It's also EXTREMELY useful if your group likes to rehearse for a few hours before an actual performance.)