In reply to Jerome Curoy:
Speaking as someone who was mostly a boulderer for ages, and then got into trad in the last couple of years:
It definitely helps to go on some sort of trad course for a few days (if you haven't already), just to get a solid grip on the basics.
Then it helps *hugely* to climb a lot with people who have much more trad experience than you do, and explicitly ask them for feedback and constructive criticism (on gear placements, but also on belay-building and everything else).
Practising gear placement at ground level as other people have suggested is also a great idea.
I have a theory that boulderers and sport climbers who get into trad are potentially a very high-risk group as n00bs because our strength and movement skills can waaaay outstrip our gear-placement skills.
So, while you're getting started (and especially while your gear keeps falling out ...), I think it's a smart idea to try to keep a nice big margin between what you're trying to lead and the limits of your physical ability (i.e. the stuff where you might be able to do all the moves fine -- but not protect yourself while you're doing it).
If you feel you need to test your movement skills to the limit and check that your strength is still there, that's the time to get the mat out.