I can do the second:
When someone buys Possession, that is a good time to spend all of your coin tokens!
Ok, I will take a stab at saying something more profound.
Coin tokens can be:
1) Treated as virtual coin.
2) Used for smoothing.
3) Hoarded for big buys later.
Virtual Money. If you are playing an engine that produces X tokens every turn (on average), it's like you are producing $X in virtual money every turn (on average) in addition to whatever else your deck does. You can count on your overall buying power increasing by that much every turn thanks to coin tokens. Your deck can usually buy a $3 or a $4, but also produces a coin token a turn? Then count on being able to buy a $4 or a $5. Take that into consideration when making your buys.
Smoothing. Is hitting $2P the same thing as losing your turn? Is getting a hand that produces $7 only as good as one that produces $5? Consider saving a few tokens to make up the difference! How many tokens you need to have on hand depends on how critical the gaps are, how many tokens your deck produces, how many you plan on using every turn for virtual money.
Hoarding. Maybe the goal is to produce a large number of coin tokens and go for one big mega-turn (don't forget you need buys!) which ends the game. In this type of strategy, coin tokens to some extent function as a delayed Bridge or Horn of Plenty turn. When to pull the trigger and make those buys depends heavily on what you expect your opponent to do, too.
Pitfalls:
Your goal should be to spend all your coin tokens by the end of the game. If you are not hoarding and you find your coin token total creeping up, 6, 8, 11, 15,... then you shouldn't be so stingy! Buy better stuff! You need to be treating more of your tokens as virtual money and bumping up your purchases to the next level.
Don't feel like you have to use your coin tokens for their intended purpose. The coin token you start Baker games with does not have to be spent on the opening. The two coin tokens you get from Butcher do not have to be spent on remodeling right now. You can spend 0, 1, or even 3 or more, instead! The coin token you get from Candlestick Maker does not have to be saved. Sometimes you need to suck it up and treat your CM as a Copper this turn. Think about what your overall strategy is and how coin tokens fit into that -- don't think about where the coin tokens came from.
You don't need to trash a card with Butcher in order to bank two coin tokens. Also, you can gain the same card you trash with Butcher without using any coin tokens (e.g. Province). Use this to your advantage.
And I'm not kidding about Possession.