I've written quite a lot of sex scenes (admittedly only in fanfic). So: less is more - allude to rather than write like a gynaecology textbook. If you want to make it erotic (you may not necessarily want to do that if your interest is character development within a general novel) then 90% of the work is actually done in the build-up before the first item of clothing comes off. In fact, that's probably a good guide for the character development you're aiming for; given that even if you were going for eroticism, a lot of it is in the build-up, you could probably apply the same to whatever character traits you want to establish.
The other thing is that it's like writing any other action scene (e.g. fight scenes, chase scenes, accounts of sports contests). The "rule of thirds" applies. Two thirds action, one third emotion. You need to get us inside what's going on in the character's heads (in a show, not tell sort of way - and PP's point about perspective is well made).
Also, I think the thing to do is make sure you write it the right way round. I spent ages with one scene (not a sex scene, but one where I wanted to get across two people developing an instant antipathy on first meeting which was going to have far-reaching plot consequences). I spent ages writing the most wooden crap you can imagine, trying to "contrive" an argument. Then I realised I was doing it entirely arse-about face. Rather than "They have to argue, now what can they talk about?" I should have been thinking "What would they, given their area of mutually overlapping interests, be talking about? Then how can I introduce some nasty, simmering under-the-surface digs into that natural conversation?"
I think the same thing could work for a sex scene. Think about what two people with their personalities would naturally do in that situation, and rough that out - then add in the extra bits which will establish the character points you want to draw out of it.
And finally, write it. Put it on one side. Come back to it in a month or two. Possibly even (just as a writing exercise) write other sex scenes for them which you have no intention of putting in the final novel, just to get comfortable with the whole idea. And remember, explicit sex, or even soft-focus sex, isn't compulsory if you find you can't get comfortable with it. There is probably a way to get the same point across without writing the scene if you find it really doesn't sit comfortably with you.