Tips for speeding up MacBook Pro

Lately I feel like my MacBook Pro (late 2011) is slower than I remember it being. Especially when opening an app. Even opening 'system preferences' takes a good 10 seconds which seems like an eternity. I try to not leave many apps open in the background if I'm not using them, usually it's just Mail, iTunes, Twitter and Chrome. There are times when I have other apps like iPhoto or Photoshop open but I expect those to slow down my Mac a bit.

Any tips or suggestions to help speed up performance? I should mention that my Mac only has 4GB of RAM and a 5400RPM HDD. I would like to upgrade to a SSD but right now it just isn't in the cards.