Always do some walking meditation before sitting, especially if you know you're feeling sleepy.
Do it anyway. Even if you drop off to sleep completely, still there is something that happens when someone takes up a seated meditation pose... a sort of 'learning' that has nothing to do with words. I'm not advocating an inattentive meditation practice, but if you are tired, perhaps sleeping is not so bad.
Another teacher recommended a 10-20 minute nap before meditating.
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