General Antonyms:
* Increase
* Enhance
* Amplify
* Grow
* Expand
* Augment
* Strengthen
* Magnify
More Specific Antonyms:
* Flourish (if referring to growth)
* Prosper (if referring to success)
* Strengthen (if referring to power)
* Brighten (if referring to light or mood)
The best antonym will depend on the specific context in which "diminish" is used.