I prefer to have my dogs on a leaner side. It's much easier on their joints for agility (well, in general really). However, easier said than done I've got two Shelties who need to lose weight. We always get comments about how thin Keeva is, but I think she's at the perfect weight. I would probably put 5 pounds on the Boxer and leave it at that. She doesn't look too terribly thin.

We had to put weight on Kaedyn when we adopted him. I did so by feeding him the puppy formula of a high quality kibble. It did a nice job of slowly putting on weight. Some others have tried "satin balls" (recipes on google), which seems to be a popular method for putting weight on dogs. I've never tried it on my dogs though.