The safest premove is when the move is illegal unless your opponent does what you expect.
For example premove your rook on top of your other rook... it's only legal if your opponent captures your rook (and now you'll recapture).
Premove your king to a square where the opponent's queen will attack it... it's only legal if they move their queen away.
Less-safe, but still common premoves are for example premoving development (or similar) when the position is calm. For example if the last few moves didn't involve any attacking or defending, and you want to maneuver your knight from b1-d2-f3-e5, you can probably premove Nd2. Then if the opponent hasn't made any aggressive move, you can premove Nf3 etc.
In general, if your opponent is able to play ANY capture, and your premove isn't a recapture, then you're being a bit risky. If you play too many risky premoves in a row, some players will try to punish you by making a crazy move.
Once, I premoved to capture an opponent piece, but they moved it away, and a premove meant it was to be moved no matter what. So, I blundered a rook and resigned. The same thing happened in my next game. I was premoving a queen capture, and they defended the piece, so I blundered a queen and resigned. After that, I went on a bit of tilt and lost about forty rating points. Do you guys have any tips or premove blunders in your games before?