I have found that In-Play betting can be very profitable, at least since I started playing In-Play seriously at the start of the year.
The way I have been looking at the games is trying to figure out which way the price will move on a particular game (usually NBA as there tends to be more liquidity) and back or lay accordingly.
Then once the game kicks off, it is a matter of either waiting for the price to go your way and taking the other side. Even if the line doesn't go your way you can usually trade out of your position and worst case scenario you can recoup some of your stake back and end up with a much smaller loss that you would have had at the start - though often you can get yourself a green book and lock in a small profit whichever team wins.
If does take a bit of getting used to I found it was well worth testing the waters by just using chump change for a little while to begin with to get a better feel for general in-play trends etc
The trick is being able to distinguish when the game state is changing from one team to the other and getting in quick - if you can get a good read on a game, then the earlier you spot this then the better price you will get.
I have not been without some upsets though, usually by getting greedy and waiting too long to trade out of my bet and making a bigger margin - when I had already had the opportunity to guarantee a small profit either way. If you can set yourself a target that you are happy with and stick to it, then in the long run you should come out on top - it will just seem like a grind to begin with...