I just hold the base super tightly and then wiggle the lid side to side before I wiggle it slowly open (if I can get it open with a circular wiggle, that's best). The only time I've ever lost product in this method was when the lid was loser than I expected it to be and it popped open super fast, so definitely check out the seal first.
And by "dealing with clamshells" I do hope you meant how to open them cleanly, otherwise I'll feel silly.
/emoticons/
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ETA: Closing is basically the same procedure, but in reverse, if that makes sense? Basically just avoiding expelling excess air, so close until lid touches base then wiggle down.