Here is my approach...
I practice on my own wrist to keep the mechanics in check. I put the watch on both ways and will experiment with putting it on tighter and looser. I also have a few watches and keep mixing up which one I'm wearing. Practicing with your hands behind your back, over your head etc also allows you to try approaching it from different angles and such. If you keep mixing it up, you'll be able to confidently steal from anyone.
In terms of doing it to an audience, I often do so as a part of a trick. If the steal fails, I still have a climax within the content of whichever effect I'm doing. I don't do "fake effects" that fail with the coin not moving to the other hand. I put it into an actual effect with a real ending so the watch steal is often an added kicker and not necessarily the trick itself. If I did a perfect steal with a great spectator, I could always choose to reveal it later and load the watch somewhere etc. to let it play by itself after the current trick was done with.
When you do any kind of a steal, be it a watch or anything else, you will get caught the odd time, especially in the beginnings. The first thing to accept is that you are performing and the audience accepts that fact. If they like you, they'll let you go anywhere with it. Knowing that you will fail and it's OK will help you to stop worrying about getting caught. Think of getting caught and an opportunity, not a loss. If you get caught you can take advantage of the by-play opportunity by joking about how tight it is, that you lost your hand and this one was a donor from a con artist - whatever suits you. The other thing you can do is say you weren't trying to get it, but that you'll show them how it would look in slow motion if you did. Take it off deliberately, get the laugh and do the easier steal after putting it on for them.
Also, don't lose sight of the fact that a failure can also result in everyone's attention on the wrist. If you've got other steals or moves you need to do, this is a great time to go for it.
When I wanted to start doing steals from pockets, I decided to mix it into a cards across type routine. The effect of some cards moving from one guy to the other is strong enough on its own so I had that security blanket. Over time, I worked in my steals while looking for an empty pocket for them to put the cards in. If I didn't get anything, fine. If I did, I had a strong kicker after showing the cards having moved ("While I was in there, I also grabbed your wallet, watch etc."). I still do it this way in my stage show today and I love it.
I hope some of that helps!