OGA,
Since you've posted your question on the US stocks area, I assume you are trading 5000 - 20000 shares.
To be brutally frank, you really shouldn't be trading that sort of position size if you have a slippage problem as you are not in as much control of your money and postions as you should be.
Are you using direct access into Nasdaq & NYSE ?
I hope you are not trading 20k of GOOG.......
Richard