Microsoft's Patterns and Practices group recently released a whole bunch of guidance on SharePoint development.
There's a heck of a lot of stuff there to digest.
Additionally, there is a tool to make sure you are calling Dispose on your SharePoint objects - http://blogs.msdn.com/sharepoint/archive/2008/11/12/announcing-spdisposecheck-tool-for-sharepoint-developers.aspx
For those of you that work with SharePoint via PowerShell, Neil Iversen (http://www.justaddcode.com/) has a nice way of keeping track of all disposable objects created in a powershell session. http://www.justaddcode.com/blog/2008/11/12/devconnections-2008-automating-common-tasks-with-powershell/
Also, here is a peek at what is coming in VS2010 for SharePoint Developers http://blogs.msdn.com/sharepoint/archive/2008/11/10/visual-studio-2010-tools-for-sharepoint-announced-at-teched-emea-developers-2008.aspx
--Michael