This guide is mainly targeted towards developers who are interested in building their own apps, but others can probably benefit from some of these too.
Tip 1: Screw IE6. Supporting IE6 can be a down-the-road feature (or v2.0 feature, however you want to call it) but for now it's going to consume much more time than it's worth. Too many web companies say, "oh, but the 10-15% of people still using IE6 are so important to me that I need to double my front-end development time just to make sure my site works in their browser."
Tip 2: Choose a technology that is quick for development. Anything with a build cycle is out - one of the major issues I have working with C# these days is that I have to compile my code and restart the app, which takes long enough to break my concentration and I have to spend time re-focusing before I can be productive again (it doesn't help that I'm a bit of a space cadet and it really doesn't take long for me to find something shiny and distracting while the code is compiling). With stuff like PHP, Ruby, Python, etc. I can make my change and do a quick Alt-Tab + Ctrl-F5 to see the updated change.
Tip 3: Use other people's code. Package management systems (Rubygems, PyPI, PEAR, etc. - is there something like this for jQuery plugins?) have many many libraries available for you to use that are built by guys who have faced the same problems as you and have made their solutions available so that you don't have to reinvent the wheel. In economics it's called capital accumulation, and you're much better off using it.
Tip 4: Use low-risk hosting. The difference between low-risk and low-cost hosting is that if your idea turns out to suck (like most of mine do) then you don't actually lose anything, where a low-cost host will charge you say $4/month regardless of what happens. An example of such a host is NearlyFreeSpeech.NET, who doesn't charge you for hosting if nobody goes to your site. It is free to sign up and create sites, and you get charged solely on the bandwidth that you use - there are a couple gotchas, I've written about them before if you want more details.
In short, you can pay as little as $5 for the hosting in the initial stages, which is less than the cost of your celebration beer when you launch your site.
Tip 5: This one is going to be a bit touchy for some folks, but it will help you get the app launched sooner. Use Facebook Connect as your authentication system. Seriously. While not everybody is on Facebook and others have privacy concerns and don't really want to sign up on sites that use Facebook Connect, there are two good reasons from a business perspective to use it for your site:
1) You don't have to build your own authentication system (or if you are following guideline 3, you don't have to integrate a library like devise to handle it). You'll probably still need to keep a users table in your app, but you won't need to handle the annoying aspects of password retrieval, profile pics, friends management, etc.
To support Facebook Connect with PHP is simple:
<?= $facebook->getLoginUrl() ?>2) Free advertising and market feedback. When people use your app, you can post stuff to their wall saying something like, "so-and-so did X on MyAwesomeApp.com!". Their friends will then see the post and (hopefully) say, "whoa sweet, let's go check that out." You can take advantage of Facebook's already-established network for marketing, or for learning what people have to say about your app.
If your app goes anywhere, then you can start thinking about supporting non-Facebook accounts. But for your initial launch you want to do the least possible work, and using something like Facebook Connect will help you achieve that goal.
Of course, some of these tips don't always apply: if you're building an app targeted toward businesses then Facebook Connect isn't really an option. I'll repeat myself: it is important for you to judge how these points might influence the way you are doing things, and to act accordingly.