1. Not aiming high enough: If you don't ask, you don't get. In other words, ask for more than you expect, or you'll get less than you deserve.
2. Doing their job: Once you make an offer, wait. The other side has three choices: accept it, reject it, or make a counteroffer--which means you can negotiate some more. If you better your offer before the other side answers, you are giving away the store. They'll label you a wimp, then wait for the perfect moment to really start grinding you down.
3. Not knowing what you want: Do your homework. Don't waste your opponent's time. Put together a checklist (for your eyes only, of course) of what you must have, would like to have and can do without.
4. Making concessions before you've seen all the demands: If you want to be sucker punched, make concessions piecemeal. Not only will your opponents hound you for more--they will out-leverage you with an eleventh-hour demand you cannot refuse. Get demands out in the open first, then get them in writing. Once you have a bird's-eye view of the battlefield, you can make concessions, but only if your opponent takes some demands off the table, too.
5. Not flinching: If there is one negotiating tactic you should master, it's the flinch. Open any book about negotiation, and you'll see this on the first page: Never accept the first offer.