If you cook, try using more spices and salt. Doesn't have to be a lot more but more than you're used to. If that's not option for you (for various reasons), I would suggest choosing calorie dense foods in your diet. Say, if you have a favorite type of nut, eat lots of it. I can go through a pound of pistachio - although I shouldn't. There's a thread on bulking tips in the beginner's forum, you should look there if you haven't already.
Exercise also increases appetite. Morning activity (maybe a walk) before breakfast is said to increase your appetite during the day. Drinking a cup of green tea first thing in the morning has a similar effect on me.
If all else fails, and you should really make sure that all else failed, try going for junk food with protein in it - pizza, hamburgers etc. I don't know anybody who can't eat a good amount of these, unless they're extremely afraid of getting fat(or self aware). Although I think the other things suggested should work.