How do I protect IP created by contractors and others before I incorporate?

The company line answer to your question is to file a provisional patent application that covers as much of your invention as possible. Do not worry about how well-written your app is and include as much content as possible (being wary of the total number of pages limitation) as whatever material you include will form the super set of what your actual patent spec eventually becomes.

Going further, you can really lock down your contractors with a CAPRAA document. There are templates online or, if you send me an email, I can send you my template.

The reality is that your protection is only really as good as your lawyers and your ability to pay said lawyers. There is no good answer here but you need to work hard to find people you can trust OR use a very compartmentalized approach wherein people build tiny units and only you have the whole picture. This latter process is harder but incredibly good at protecting your IP.

Finally, becoming "incorporated" is extremely easy and you can change things for an extended period of time - simply go to http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Apply-for-an-Employer-Identification-Number-(EIN)-Online and get your free EIN - I recommend using an LLC structure upfront.