Do some pruning, members with X posts or less, old threads and posts etc etc
Admincp>>Threads & Posts>>prune
Admincp>>Users>>Prune / Move Users
More pruning options:
Admincp>>Usergroups>>Prune Social Groups
Admincp>>Statistics & Logs>>Control Panel Log
Admincp>>Statistics & Logs>>Moderator Log
Admincp>>Statistics & Logs>>Scheduled Task Log
Admincp>>Statistics & Logs>>Transaction Log
Optimise and repair tables
Admincp>>Maintenance>>Repair / Optimize Tables
If moving you could also remove all mods and reinstall once setup on new server.
Remember to Update Counters after
Admincp>>Maintenance>>Update Counters