Buying accessories adds value to your pinatas, as well as their "joy level". If they are overjoyed, their value gets a little boost. If the pinata isn't overjoyed, purchase some joy candy and direct the pinata to it. It will eat the candy and the joy level will rise.
Sending them away for factory challenges also adds value. There are often challenges that pop up asking for "a single pinata, preferably a valuable one". When this happens, always send away the one you are trying to value-boost. When it's returned, it'll be worth more. I picked a pinata with a fairly high base value, maxed him out with accessories, then mailed him to myself and left him at the post office until the challenges popped up, then I'd retrieve him...send him away...then repeat the process. Keeping him at the post office keeps him safe so you don't have to worry about him getting sick, fighting with other pinatas, etc ;~)
You obviously need to do this with one of the more rare/valuable pinatas so you don't have to work as hard to get the achievement.