The recommended weight to use for eccentric training is generally 110-140% of your 1RM.
Also, contrary to what wes and others writes in this thread, I am of the opinion that eccentric training is very safe.
Most injuries actually happens during the concentric phase of a lift.
The only problem, IMO, with eccentrics are that they are very taxing and should be used sparingly.
Three sets of three reps in the middle of a benchpress workout (start off with regular benchpress, then the eccentrics, and finish off with a few sets of close grips perhaps)
Either use eccentrics for 2-3 weeks in a row and then do 6-8 weeks of regular benchpressing with no eccentrics.
Or try do eccentrics every second or third week.
When doing the the eccentrics, you need at least one very strong spotter, preferably three.
The guy who spots you from behind should keep his hand near the bar the whole time, and be prepared to give you a little assistance in the lowering portion in weak parts in the 2nd and 3rd rep especially.
He also needs to pull the bar as hard as possible once it's down.
It's not supposed to be some kind of eccentric/"lift as hard as you can-concentric". Ideally, you should only be guiding the bar on the way up.
That is only possible if you have three spotters.