Well if you mean practice as in just playing games, then you would have to play against much higher rated players to excel and also it is best to study than to just play. Study first then practice what you have studied in your games.And analyze your games,without a chess engine at first, wrighting down every variation you can possibly wright on every single move. This will take some time but it will help you see where you are weak,such as board vision,tactics missed, Blunder moves.etc...Also will let you see what stage of the game is your weakest(opening,middle,end)After you have went through the whole game and done all that wrighting and you think you are fully done with all your calculations then you can run through the game with a chess engine and see what variations it sees plus you can log your variations and see how it feels about them..This will help you fine tune your game.
Getting Worse with Practice
How can 2 years of regular chess cause my rating to fall, ever faster? They say, "practice makes perfect" but my rating has fallen steadily from 1506 to 1156 and I lost 9 of my last 10 games! I also lost my Queen in the last three games. Advice welcome, but I'm particularly interested to understand how practice can actuallyone's performance!
After reviewing the 2 games I could access on your home page, I can share with you the following:
In the first game you allowed your Q to be captured because you captured a N on d5 with your N on f3 which pinned by the Black B on h5. The piece behind the N was your Q. and completely missed or forgot that the Black B of your opponent on h5 could capture your Q. You lost your Q in that game. In the 2nd game against the same opponent you lost your undefended B on d3 to his Black N on f4. You completely ignored the threat of capture of your undefended B, and moved Bg5 instead.
First, you need to ask yourself the following very important question before you make a move: What is my opponent's threat.
If your are interested in learning more please let me know.
Thanks to everyone. I accept that one will not improve merely by playing. It seems one has to put effort into study to discover how each problem arises, and how it will be avoided thereafter. But I am still surprised that I did not make those errors in the past, only after playing more chess.
I am pretty much in agreement with transpo and giakoum. Giving pieces away for free like that is your #1 problem at the moment. I also make silly mistakes like that sometimes, and 99% of the time it was because I clicked that Submit button too quickly and did not take my opponent's counter-threats into account. You see what you think is a decent move and the thinking stops right there. 
The other 1% was due to the fact that I had a glass of wine (or two) before I logged on to look at my games. 
I teach chess in the school system as well as privately so I analyze a lot of games. I went through your last game to get a sense of where you might be having problems. The biggest issue, as pointed out by transpo and giakoum, is giving material away. Here are some simple suggestions to keep in mind as you play. First, look at all of your opponents pieces before considering any move. Notice the squares those pieces control. If any of your pieces are under attack, ask yourself, "are my pieces protected?" If not consider defending them or moving them to a safer place on the board. Next, come up with not one good move but three possible moves. Then determine which move is the best. Often we miss the best move because we choose the first move we see.
You lost material early on, such as the Queen, because you were focusing in on the center of the board, trying to control it rather than looking at the entire board. The Bishop pin on your Knight on f3 should have been broken before moving the Knight. However, there is a mating trap that involves the sacrifice of the Queen in that position, but it requires it being completely set up before attempting it (I thought you might have been sacrificing the Queen for a big mating attack). By set up, I mean having your pawns and minor pieces in position to start the mating process prior to giving up the Queen.
I would suggest examining all of your opponent's pawn and piece positions prior to moving and coming up with at least three moves before considering moving a pawn/piece.
Also use the opening principles to guide your opening game. The biggest problem I saw was hanging pieces. This can be fixed by examining your opponent's position and determining which of your pieces is under attack. Try working out with computer software set at a higher level. While, you may not win a lot of games against the computer, you'll become a stronger player. If you want I can do some free analysis for you and break down the problems move by move. Just let me know.
How can 2 years of regular chess cause my rating to fall, ever faster? They say, "practice makes perfect" but my rating has fallen steadily from 1506 to 1156 and I lost 9 of my last 10 games! I also lost my Queen in the last three games. Advice welcome, but I'm particularly interested to understand how practice can actually <impair> one's performance!