Barrel placement

I've made four maps so far and barrels aren't emphasized much. Any tips for barrel placement? I find myself using them in a somewhat forced manner and I don't want to do that.

Well you could maybe place them near some monsters far away enough that the player when he sees the barrel and monsters he has a choice, whether or not to wait, for the monsters to run near the barrel or just simply shoot the monsters. I'd recommend just playing around with it maybe put it far away enough where it could damage a strong enemy but not kill it, giving the player an edge.
But it also depends on the maps being made too, and whether or not the barrels fit the level.

I don't think it is good barrel placement if the player has to just wait. A little maneuver should be required from him, maneuver that is dangerous in some way, but is worth doing.

I wouldn't get too self concious about it. There isnt a whole lot you can do with barrels besides blowing up monsters, or blocking a path, requiring the player to wake up monsters. They're not very dynamic beyond that unless you want to create a wild chain of explosions ala barrels o fun, which will appear kinda forced anyway.

The best you can do is anticipate where the player will be entering a room from and putting the barrels somewhere between the player and the monsters. Ideally near high ledges or flights of stairs for added effect. As a player, its rare ill kill monsters with a barrel explosion and be like "wow. The mapper obviously wanted me to do that." Unless you like... box a hell baron up in some barrel prison. I guess you could soften up the forced feeling by placing some barrels in random decorative locations so the player gets to pick which ones would be most effective.