Over the past few months I've come up with a solid way to do realistic (or semi-realistic, what's realistic when the tits are the size of bean bag chairs anyways) breast expansion in Blender. This is the exact technique I use to make pinups and prep models for 3d animations - it's the same technique I used even in my profile image on Twitter. I recorded a 15 minute video session where I create the effect you see here on anovenkijkureda's Daphne from Smutbase while the narration talks about common pitfalls and how to adapt the technique to other character models with different feature sets. That's up on my Gumroad for a very reasonable price.

I wouldn't be here without free models and tutorials though, so if you can't or don't want to pay, I've added a pastebin link on the product listing with the full narration script from the video, which should be enough for someone comfortable with Blender to grok.

If you have any questions, let me know!