If you're using POP you can
a) delete each mail after having read it
b) delete each mail after your mailbox exceeds 100
c) delete packagewise (that's what I'm doing with my POP accounts, after 2 weeks I delete the lot)
If you're using IMAP you can
archive all mails you don't want in your phone to the local folder in Apple Mail on your OSX device. This will remove all your mails you don't want on your phone. You still have a copy on your OSX device.
Delete all mails in your trash (removed 0,5 GB last time I did it.
Wait for iOS 8 out in September
I'm using Macs since 1996 (MacOS 7:)). There's always been ups & downs but never hit me hard enough to change to incapable systems.