How do I trim Time Machine backup history?

Time Machine will do it for you automatically. http://pondini.org/TM/12.html

There doesn't seem to be a way to delete multiple backups via the starfield. You could use Terminal and tmutil delete to quickly delete multiple backups:

1. Make sure the backup disk is mounted (entering the starfield and then exiting it should work)
2. Navigate to the folder that contains the backups in the Finder. (I don't quite remember, but I think you get there by double-clicking on the backup disk on the desktop, then Backups.db, then a folder with the name of the computer being backed up.)
3. Open Terminal and type sudo tmutil delete (the space afterwards is important!)
4. Select all the backups that you want to delete and drag them into the Terminal window. That will automatically insert the filepaths for you. Then press return.
5. You will be asked for a password. That's your account password, the one you use to log into your computer. The characters won't echo, not even as dots or asterisks, so just type it carefully and then press return again.