Well, I've done dips before… but I hardly do them at the club. And the main reason for this salsa tip is because I see a LOT of beginners trying to do dips in salsa, when they are barely trying to get the basic step down.
Doing big dips in a club is a very dangerous game. Now, there is a difference between doing a small dip, and a large elaborated? dip. Small dips are fine, the lady doesn't go down much (and doesn't feel like she has to break her back). However, with large dips – its tricky and risky.
The floor is crowded. One time, when I decided to do a dip to this cute girl I was trying to impress, it ended up being that another guy next to me was doing a dip as well. For some statistical reason, we ended up doing the dip in opposite directions (and similar dip) so that the girls ended up hitting their heads. Yeah.. real smooth.. (sarcasm). Well, then I realized that when the floor is this packed, you will probably never want to do dips like that so you don't accidentally injure your follower. For these, you might as well do a standing up dip (not sure what they are called).
Don't operate heavy machinery. It will be the case that the ladies might not know how to perform a correct dip. In a good dip, the lady basically holds her own weight (and does not launch herself on the floor for you to catch her and support her – while not breaking your lower back). Some ladies think that its up to the guy to magically work against the laws of physics to perform the dip – and trust me, it doesn't work. Either you'll get injured, or she'll be on the floor. So, before you try a dip, make sure you feel like you are with a comfortable dancer that understands dips. Like in the picture shown on the right – that is not a good dip.
Find a good spot in the song. If you are going to do a dip on a given dance in a given song – try to do it to a song that you know that might have an accentuated pause, or drawn out instrument piece. Why? Well, you can't just do a dip anywhere. If you try to do a dip in the regular beats of the song, it looses some of the feeling of how your body moves with the music. In addition, you might end up doing the dip either too fast or too slow to maintain synchronization with the song, which will make the dip look sloppy.
A hand in head support. If you are doing a dip, where the lady's head is going down – my biggest tip is to use your free hand and give her some head support by placing your palm behind her head (as if it was like a pillow) so she doesn't feel a type of 'whiplash' effect when doing the dip. Trust me, it will make the dip feel 100x more comfortable. (again, thats why I say that the picture shown is a bad dip).
All in all, dipping is about one main thing: making the dance end with graceful closure. How to make that happen is two things: make the lady comfortable when dipping and make her feel secure. Remember, the dip is for her, and not for the audience. Simple, comfortable and elegant dips are the best.
[UPDATED] The easiest way to get into a dip is to do a cross-body lead with a turn, and into a simple dip.