What field is it in? For ours ( http://feefighters.com ), which is b2b app, we have a 6 pronged attack which works pretty well: 1) SEO - people find us when they search for credit card processing and lots of long tail keywords. This is a long term thing but completely worth it. Make sure all of your pages are optimized for SEO. 2) SEM - we were buying keywords on google/bing/yahoo. Facebook has been less effective fo…
Taking my own advice... One of the things that brings people to http://hubski.com (a social aggregation site) is original content. I like to write, and I found that when I wrote on hubski, that content brought quite a bit of traffic. If you are creating in the social space, IMHO you need to think of your site as a restaurant. Above everything else, you need to have quality food. You can get tons of traffic, but if you don't have quality food, people won't come back. It's easier said than done, but I found that contributing my own original content seemed to provide a value that few other things did. I've come to the opinion that quality is everything. The little things count. Be tough with yourself, and listen to criticism.
Ask yourself: "Do I like my app? and What don't I like about my app?" everyday. If you love your app, it will be infinitely easier for you to promote. Make sure you love your app before you start putting serious resources into promoting it. If you love your app, traffic to your site will likely be more effective.
BTW, this post is at #3 samrat. This would be a very good time to tell us what your app is! :)
EDIT: Another thing to try: To get some objective feedback on first impressions, you might want to try stumbleupon. People can vote on your site when they stumble it, and you can look at the average score of your site. IMO I don't think that SU is an effective way to advertise, but that does provide a measure for first impressions. If your score is low, you know you need to work on your landing page.