I haven't got form 16 from my previous employers

Yes it is mandatory for the companies to issue form16 to the employees.  But as you are no more working with the previous companies you need to communicate with the HR dept to get your form16.  Did you communicate with the respective HR's of your previous companies.

without Form16 you cannot prepare your I-T return.  You need to do this before 31st as this is the last date of filing your return.  however if your TDS made is up-to-date then you can file your return aftr 31st july also. But ensure if you do not have tax liability after considering the combined income from all the companies during F.Y. 2011-12 else you will have to pay interest.

Hi...

  You cannot file the return until you get form 16 OR amount should reflect in Form26AS which can be downloaded from 'TIN-NSDL.COM'. In case if you unbale to receive the form 16 from employers, you can cross verify the amount showed in form 26AS along with amount deducted in payslips. If total amount is matched, you can file the ITR. 

Regards

Anand.