Most people use:
http://bitcoin.clarkmoody.com/And as a rule of thumb, if your buying 10+ BTC, don't try to bid above any offers of volume less than 2.1BTC.
(EDIT: I added the .1 as I've just noticed since posting this some of the "Scaffolders" have upped their offers to lots of 2.0BTC bids rather than 0.1)
If your buying 100 BTC, don't try to bid above any offers of volume less than 15-20BTC.
If your buying 1000BTC, don't try to bid above any offers of volume less than 100BTC.
The two guys in my screen shot had just put in orders for 300BTC and 1000BTC at $142, really they should have gone in around $138. It was at $138 for the last 12 hours and as soon as they did that we've now been around the low $140s ever since. It's entirely their fault for the price rise lol. You can see a big green bar on the graph at the exact point their two trades happened.
Lastly, don't get into a bidding war with someone unless there really is a good reason in the news or something that you suspect the price to increase very soon and so you're actually wanting to buy FAST. Just put in your offer and and wait a while (an hour maybe). If nothing happens, raise your offer a bit.
Final point, the Mt.Gox trading floors in the (US and UK at least) have been extremely quiet the last 48 hours. I get the feeling maybe everyone is a bit scared of buying or something because of the big drop. So take that into consideration if you are trying to buy right now, it's very quiet so could take a while for someone to come along who's wanting to sell their bitcoins.
After all, it's a negotiation. You are supposed to barter/haggle
The bigger your order (number of bitcoins your wanting to buy) the bigger your negotiating power