Don't be embarassed about asking people to review you favourably - think of it as asking someone to recommend you to a friend. If you were a plumber or electrician and someone thanked you for doing a good job you wouldn't feel like an ass saying "Thanks, if you know anyone else who needs a --- make sure you give them my number".
If you help someone out and they write back to thank you, just fire them back a quick email saying "Thanks for letting me know, glad the app works how you need. If you get a minute I would really appreciate if you let others know by writing a quick review on the app store. If not, no problem, thanks for using the app". If you've provided people with good service they will want to tell other people. Most business works by referral from people who are pleased with what you've done.
