What kind of deal to offer a co-signer?

[Roy Pales]

My wife's mom co-sigined our mortgage when we purchased a 3 unit property 2 years ago. It's time to re-fi and we need her to co-sign the next purchase since both wifey and I are self employed and have variable incomes. Mom is happy to do it - but as an investment this time, not just out of good will.

What kind of terms should we offer her? I'm concerned about possible future conflicting interests, like we want to sell and she doesn't or vice-versa. 

Any advice would be appreciated.