I have found Level 2 to be helpful. A few suggestions -
1. Don't pay attention to big size showing, this is used to trick you.
2. The real big size is (usually MMs and not ECNs) showing 10 (1000 shares) or less. Look for many prints at that price with them staying put at their price. This indicates they have reserve. Look for MMs that do this alot in the stocks you watch, and trade on the same side as them.
3. Do pay attention to big size moving up/down in price, this is more likely to be real buying/selling interest rather than a trick.
4. Look for ECNs (INCA usually) crossing the market. They are doing this more often since Supermontage and a crossed mkt gets you at least an extra .01. Not much, but nice if you are scalping.
5. Watch T&S in tandem with L2. If prints are going off at the price, if the bids/offers are being taken out. No T&S prints? - they are being pulled (not real buying/selling interest).
6. If your charts are telling you something, don't do the opposite based on what the L2 looks like. Charts aren't the most reliable, but are more reliable than L2.
7. Realize that daytraders are out there that will join a bid/offer if there is a wide spread. You can bid/offer, after they join you - cancel your order and take them out. If enough people do this, maybe we can get rid of the "liquidity traders" for good!
Nasdaq gaming is great fun!