Once or twice a year have a month long donation drive where users are encouraged, pleaded with and even begged to donate even $1 to the Linux Mint cause. I have seen this used on other sites / projects with great success, with revenues of over $20,000 raised in a single month.
The most successful drives are for a specific and well communicated purpose such as:
Feed our starving developers
and so on
I don't know what the user base is for Linux Mint, but it is surely large enough to target $20,000 in a donation drive.