Everything below is based on personal experience and probably from a female mobile gamer's point of view.
① Monthly Subscription VIP with daily premium currency as rewards
How it works: Player purchases the Membership Subscription ---> Receive X diamonds (premium currency) everyday he login.
How players feel: Everyday I can get some premium currency as long as I login.
Results: since this player already made the purchase, you've locked him for at least one month. --> retention rate higher + $$$
② Take Care of both Short-term and Long-term Purchase Mindsets
Assume there are 3 IAP options:
a) Pay $2 for 20 diamonds.
b) Pay $10 for 100 diamonds.
c) Pay $12 for 1 month VIP subscription and you will get 5 free diamond everyday
Option C is definitely a more cost-effective option in the long term but players would have to wait to accumulate the amount of diamonds he need.
For players who really wants all the diamond right now, he has option a and b. For players who are not so in a rush but still want the diamond sometime in the future, he will pay for the VIP and wait for it to accumulate. Sometimes players don't want to see the situation as 'I paid $10 a day just for a mobile game'. The idea of 'I paid $12 for 30 days for a mobile game...which is just $0.4 per day' makes them feel better and less guilty and thus more willing to make a purchase.
It made me think of a joke made by a girl who likes to buy bags. She did a calculation: assume there's a branded bag that costs $500. She will use it for at least 2 years. So she only need to pay $500/(2*365)=$0.68≈$0.7 per day. This means she only paid $0.7 today to buy this bag!!! This thus led to her conclusion: WHY NOT JUST BUY IT? IT'S SO CHEAP!
Jokes aside..if you understand what I meant, you will see the 'long term' and 'short term' mental perception differences.
③ Make your Item Fancy
Let's take the above option c for example.
Let's redecorate it to be "Pay $12 for 1 month VIP subscription and you will get 5 free diamond everyday + 80% discount on any item you buy during VIP period + some fancy stuff/effect in the game'.
First of all, 80% discount? When players know that they get to buy things at a cheaper price than others, they are more likely to make a purchase. Also, since they already knew that they have the privilege to pay at a discounted price, they tend to keep thinking maybe they should buy something or else he wasted his money on the VIP subscription.
Secondly, the 'fancy stuff' cost nothing to developers. Probably it's just some entrance effect...or a golden interface...or whatever things that doesn't really matter to the game but just looks fancy.
This has something to do with the Bundle Effect --- which refers to the phenomenon that people feel they are getting more stuff with the same price but in fact the cost of the goods sold stay the same.
④ Use Notification Sign to Direct Players into the Appstore
You can put an exclamation mark on the store icon 5mins after player open your game. People tend to open everything with a red exclamation mark. Therefore, even if they don't purchase anything, at least they get to see what's there in the store. This helps to build a subconscious 'purchase' idea in the mind.
⑤ Level up the Competitiveness
If your games are not competitive at all, people won't pay.
Remember what D.Va said?
"I Play To Win."
Normally people pay because they want to be 'superior than others' or 'look more pretty/awesome'. If you don't catch these two major mental factors when designing IAPs or even the entire game, it would be super difficult to get their $$.
⑥ Learn from the Best
If possible, play some top games published by Tencent...and then you will know why Chinese game companies are so rich nowadays.
(Disclaimer: a good game doesn't equal to a profitable game. Sometimes I hate Tencent's games because this company simply has 1000 ways to drain all your money. :)