Hmm. I'm not sure I agree about selling mats in the weekend. Sure, there are lots more players in the weekend, but this also means that many more players are out skinning, which will drive the prices down. If the price at a friday is 1g50 for a stack of heavy leather (can't remember the going prices ATM), and everybody who puts a stack up lowers the price a silver or five, you will very quickly get below the 1g mark, and as long as there are players out skinning, the price will stay down or go even lower. This is just an example. The more people who put mats up, the more the price will be lowered.
From what I have observed, the influx of mats last until monday or tuesday. At this time the extra mats will either have been bought or their auctions have run out, leaving only the regular week-players as suppliers. I start selling my mats late tuesday and stop again friday. I store the mats in the bank during the weekend. I get substantially higher prices by doing this. The only example I can remember right now is with light leather; it usually goes for 20-40s, yet by putting it up at the right moment during the week, I sold them for 1g.
Also do not put more than 3-4 stacks up at a time. Spread them out, and sell a few at a time. If you put many up at a time, and the price falls below what you set it at, your mats will just be sitting there, and you'll lose your deposity. By saving some stacks, you are able to either post some more stacks, at a lower, competitive price, or save them, and the deposity, for a better time.
Are you setting a buyout price? Most people want whatever they need NOW, and will simply skip items without buyout. I strongly suggest always putting buyout on everything (except perhaps rare epics and the like, where people will really make a bid war for it).
Another important thing is the age of your server. If it is new, the prices can flunctuate a LOT, not just though the week - that is normal - but though months or more. I ended up dropping herbalism because herbs suddenly just would not sell. I tried waiting several weeks, storing the herbs in the bank, but the situation didn't change, so I decided to take mining up instead. I don't think this is the problem for you, however, as skinning feeds more professions unlike herbalism, which is only (really) useful for alchemy.
That became rather long...