I used to like Colette, although I was never cool enough to feel at home there. They are one of the two places in Paris that carry RJ, although it's never on display. Still, I bought some wonderful cotton lisle RJ long-sleeve tees there. Colette now appears to be turning into one of the circles of hell, like Harrods -- always full to bursting with the pretentious in the way that Horrids is with tourists. The water bar was nice, although now there appears to be some Italian-influenced eatery downstairs and I'm not sure they still have the water lineup. When, in 2004, their menu proudly proclaimed they were serving Starbucks coffee, I knew they had jumped le requin.
However, none of the shops so far mentioned are preppy. Are you sure you are preppy? The French preppy analog, the BCBG, would be more likely to wear British-influenced Barbours with shetland crews and tattersall shirts with cords. Old England could do you well, although it's very expensive too.
Frankly, Paris is not a good place for cheap clothes, unless you go to the rather scary cheap shops in the north of Paris or to the flea markets, in which case quality will be astoundingly bad. Used clothing is not widely available or of interesting quality, and outlets are sparse although there are some areas -- pick up Paris Pas Cher, if you haven't already -- that have off-price retailers. I'm thinking the area around the Saint-Placide metro, the rue Vieille du Temple, Mouton a 5 Pattes on Boul St-Germain and one or two places down Boul Raspail. There's also the outlet center out by the airport and ebay, which the French haven't been as quick to turn to for shopping.
I used to like Sisley (owned by Benetton) for disposa-chic when I was a student. Not too preppy, though. Quality is as bad as Benetton but their sweaters were nice.
There was an APC sale shop near St Sulpice; also try H&M; Le Bon Marche is having its final sales right now so I imagine the other big department stores are as well... The stores along Rivoli offer a mix from the trendoid to Hildeetch... the trendier stores often have very good sales.
If you're still figuring out your style, go to the Gap, Celio, Naf Naf or H&M and try stuff that interests you. If you don't like it, you won't have to have spent a whole lot of money on it.