I've played Chechoslovakia in DH Full 1.02, 1933 scenario. All you have to do is to get level 10 fortifications all along the border and keep 3 to 4 divisions in each border province (and remember, that Hungary and Romania may eventually join Axis, thus extending the front). As DeusEversor said, don't accept Munich treaty. After you reject it it will kick you out of Allies, but nothing stops you from joining them again after a few months (it took me around six requests before they agreed).
The German AI is simply afraid to launch a land attack under such circumstances as 10 lvl fort. German Stukas are pain in the butt, although if you don't move your army, your losses from them are gonna be small. After you join Allies, they're most probably gonna send some wings of interceptors to defend your air space. That's, basically, all you can do: keep defenses, research technologies, build divisions and/or factories, and wait for a moment to strike. Believe me, it'll come.
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But the biggest fun in my game started after the Americans went for the WWIII:
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Anyway, it's you AAR, and I'm sure it's gonna be interesting. Good luck!