When is it appropriate to delete requests from the PSA

Hi All

I am using BW3.5. I we frequently are getting problems due to our table spaces being full. This in general is caused by the volume of requests in the PSA. What I want to know is what is safe to delete and what do you really want to keep? Are there any guidelines to help determine this.

I don't want to delete unknowingly something that I may need in the future.

Many Thanks

Tim