Here's a bit of advice from my 4 years developing F2P games:
1) Don't scam your players. Do the introspection; how do you feel when the game is trying to rip you off?
Understand that your game may be funded by only 10% of the players, and 90% of them will play solely for free (its quite possible, even perhaps, desirable!).
If the players that are playing for free have a great time, they'll contribute by spreading the word, and getting you valuable players from the 10% you need to stay afloat.
2) Provide meaningful, but not game-breaking advantages to paying players:
Though customization items only represent roughly 2-4% of the entire purchases in F2P games, they can be considered valuable in that they don't affect the game balance. If the game is MMO, it gives the paying players bragging rights to look "involved" in the game.
If these customization items are also available to free players, albeit through grinding, it could help establish a social order within your game. Some games have managed to finance themselves solely on this model.
As per the above, you can't include balance-breaking items for paying players, but you can give them versatility.
The Planetside 2 model is especially great here as each gun is more or less the same (they are fairly balanced) but players will feel more comfortable playing with guns with faster/slower rates of fire, better/worse accuracy and damage, etc. Let them choose their playstyle, but attach a price to it. Make them understand they can kill everyone with the basic gear, but that choosing their favorite gun will allow them to match THEIR playstyle.
If you game allows specialist-type guns by default (say, sniper, shotgun and smg), make sure you put a versatile gun for sale (auto-rifle).
4) Launch and watch
It's possible that your model will be flawed day one (technically, all game's model is flawed upon release). See what people ask form, consider it, and apply as needed.
If players ask for a feature in sufficient numbers and there's a way to make it balanced, do it and sell it, it will sell. Though its often true that players don't know what they want until they see it, once they've seen enough of your game, they can have very insightful feedbacks to provide. YOUR game becomes THEIR game upon launch.
This is critical.
I'm part of the school of people that believe you're not trying to create fanatics, but rather, want to convert fanatic players to your game. The best way to do so is make it appealing to them.
Define the playerbase you're trying to get to, see what they like.
If you're making a City-building game with elements of combat, make an Evony account. Log into that game. Join a guild. Play for 6 weeks.
Notice who spends money, and what they do it for. Look at the chat what their reasoning is for spending this amount of money.
Learn and watch.
Once you feel confident you understand these players, build your game with these people in mind. Remember what matters to them, and choose accordingly.
You can't build a successful F2P game without having an intricate (psychological) understanding of your "whales" (expression that defines roughly a user group that is a minority in your game, yet constitutes the majority of your sales).
When you consider item #4 above, remember that your whales carry more weight. If a decision would not be unanimous, check whether the whales will be happy about it, and choose accordingly. While it's important to keep the free players happy, your whales are more important.
For anything more specific, you'd need to show us a concept of what you're trying to achieve.