I found a neat document from MS that details some very usefull tips. The contents are:
Best Practices - which includes sections on performance, rendering, page size, logs, parameter passing, and how to switch from a 2005 version back to a SSRS 2000 version.
Report Samples - sections on using custom code for aggregations, passing images into the report, data from multiple sources, changing the report at runtime, resetting page numbers (usefull for muti-page invoices and such), using matrix controls to make data run across the page instead of down, and creating 'Green bar' reports.
You can download the document here: