How far out are you from your mortgage app? Conventional wisdom says not to apply for anything at all within 6 months of a mortgage app, for the very reason you describe, unless the gain from new credit negates the effect of re-aging, etc.
The biggest thing that you can do for a quick score improvement is to pay down any displayed balances. Even if you pay your CC bills in full every month, if a balance due is displayed on your statement, this is reported to the credit bureaus, and it looks as if you're carrying CC balances. Pay off every CC but one several days before the statement is due to drop; let that one remaining card report $10-20, and then pay it off. Please post back if you have AmEx, HSBC, Orchard, or US Bank --they report on different schedules.
At any rate, util (amount owed divided by credit limits extended) is a huge driver of FICO scores, and it's the one area that is somewhat controllable by the consumer, if you have the cash to pay off your cards.
* Credit is a wonderful servant, but a terrible master. * Who's the boss --you or your credit?
FICO's: EQ 781 - TU 793 - EX 779 (from PSECU) - Done credit hunting; having fun with credit gardening. - EQ 590 on 5/14/2007