The commonest error is offering with the piece still in your hand. You say “draw?”, your opponent starts thinking about it, and he is thinking on your clock while you have not yet committed to a move. Backwards, and the Laws say so.
Article 9.1 sets the sequence. Play your move on the board, offer the draw, then press the clock. That way your opponent considers the offer and your move together, on his own time, which is what he is entitled to.
What the offer is and is not
It is unconditional. “Draw if you play 20…Rf8” is not an offer, it is a conversation, and the Laws do not recognise it.
It cannot be withdrawn. From the moment you speak, the offer stands until he accepts, declines out loud, declines by touching a piece with the intention of moving it, or the game finishes some other way. That is the whole reason to check the position first: I have watched a player offer, spot a winning fork while his opponent was thinking, and have to sit through the handshake anyway.
Both players must have completed at least one move for the agreement to end the game (Art. 5.2.3). And an offer made at the wrong moment, before you move or after you press, is still valid. It just risks Article 11.5, the rule against distracting or annoying your opponent.
Asking twice
Do not. Repeating an offer every few moves is the classic way to get an arbiter involved, and a warning or a time penalty is the usual result. One offer, then play chess. If the position genuinely changes character (you win a pawn, he wins a pawn, the clock situation flips), a second offer forty moves later is normal and nobody minds.
The other bad habit is offering in a lost position to a player who is short of time, hoping the interruption breaks his concentration. It works often enough that people do it. It is still ugly, and in a rated event it is the kind of thing arbiters remember. The mechanics of doing it politely are on how to offer a draw.
Grandmaster draws, and the rules that killed them
A grandmaster draw is a handshake after twelve moves of theory in a position where nobody has tried anything. Round robins used to be full of them, particularly in the last round when both players were happy with their score, and spectators hated it.
The tournament answer came from the M-Tel Masters in Sofia, which stopped players offering each other draws at all: an offer had to go through the arbiter, who allowed it only where the rules already gave a draw, such as a threefold repetition, the fifty-move rule or a position with no mate left in it. Those conditions are still called Sofia rules, and versions of them turn up all over the calendar. Plenty of opens use a simpler version: no agreed draw before move thirty.
None of this touches your club game. What does touch your club game is the decision itself, and the honest answer for most players below master level is that they accept far too many. Should you accept a draw offer goes through when to play on, and how to break a rating plateau is the longer version of the same argument.