Hi, my name is Kruno, I had several startups and several partners in them. First of all, you should know how much money founders invested in the startup, and then how much money you need to bring inside of the company and in which period. Second, if this idea is a long shot and "maybe" can be the new facebook or new google you can participate with your work to bring an evaluation of the startup on a higher level.
Also, instead of money, you can offer your network of connections (to find investors, peach an idea etc...). Deffinetley check one of the online tools (e.g. http://foundrs.com/). And everything that you agree, put on the contract!!!! this is a must! Try to be very detail because if you don't put everything in the beginning, later you will have only problems with other founders.
For more questions, you can ping me anytime.
Have a nice day,
Kruno

Your decision should depend upon the valuation of startup and your total efforts required to be contributed along with your entrepreneurial worth. You may setup a call, I can help you in this.