Besides all the diaper cream suggestions (and you may already be doing this) we had "bare bottom time" as part of our daughter's bedtime routine while she was a baby, and it helped keep diaper rash at bay. Keeping everything aired out will prevent yeast from growing in the first place, so that your diaper cream can be more effective later. We often just laid out a bunch of cloth prefolds on the floor and let her roll around for about 15-20 minutes before we took her up to bed. It was fun, we got to play on the floor with her, too. :)
And even though she's 2 now, we still have to resort to bare bottom time on occasion as she'll still get a yeasty rash sometimes. So we'll let her play on the deck with just a dress or t-shirt on, even just once a day, and it helps.
Good luck, it's hard to see that rash on your little one!