I try to wait until an app goes on sale -- or better yet, goes free for a while -- before buying it.
So, AppRecs now has a Price Drops feature. On the home page, you can the most popular apps that have dropped in price in the last day or two. For iOS, you'll see ones that have dropped in price as well as ones that have gone free. For Android, you'll see just the price drops -- Google Play reportedly doesn't allow apps to temporarily go free.
Want to know when an app you're interested in goes on sale? Add it to your watchlist, and we'll send you an email notification.
There's also a dedicated Price Drops page where you can see the complete list.
Along with the Price Drops page comes another useful feature. On the detail page for any given app, you can now see if the app's price has dropped recently. If it has, consider adding it to your watchlist so that you'll be notified the next time it happens. If it hasn't, maybe just get it now since there's might not be much sense in holding out for a lower price.