I think the real key lies in the serve. To serve and volley on clay, you need to have a good kick serve. This serve will allow you a little more time to get to the net, and its high bounce will often time elicit a weak return from the opponent. Big flat serves are really a disadvantage for the serve and volley player on clay. The slice serve wide on the deuce court is useful, but you can't use it too often.
Slice is very useful on clay.
Here again, pace can actually be counterproductive. Your opponent will have to lift the ball and the lack of pace will make this task more difficult. Angles are the name of the game on clay. The sharp crosscourt shot that lands near the junctions of the sidelines and service lines will pay big dividends on clay. You may not own this shot, but on clay it is much easier to hit because the ball moves more slowly and bounces higher.