Maybe you have heard about all the great stuff for Outlook that is coming in Office 2007 Service Pack 2.
The good thing that you don't have to wait for the service pack to leverage all the goodies, there is two hotfixes that do great things for your outlook.
First there is the big one
and the smaller one
and some info what they contain for goodies
Read those KB's and you will not want to live without these fixes.
You really need these patches, they contain so much good things that you simply cannot do without them. Number 1 is the big improvement for big OST and PST files. The performance gain is really outstanding if you have a couple of GB in your files.
Number 2 is the changed behavior when outlook closes that gives you fewer corrupt files and faster shutdown of outlook.
Number 3 is all the good calendar bugfixes.