For the Rational Developer for i V9.1 trial (http://www.ibm.com/developerworks/downloads/r/rdi/), there are two options. One is to download the zip files of the product, unzip them, and run the install. The other is to perform a web install. This involves installing IBM Installation Manager which is configured to install the product from IBM servers.
For the web install method, a technote (http://www-01.ibm.com/support/docview.wss?uid=swg21667734) was published with manual steps you can perform to accomplish it. Recently, the technote was updated with an automated procedure for performing a web install of the V9.1 product on Windows. This procedure provides a zip to download (which includes IBM Installation Manager), unzip, and run a .bat file. If you already have IBM Installation Manager installed, it might be faster to still use the manual procedure.
If you are planning on installing on multiple machines, then you should consider the options available at http://www-01.ibm.com/support/docview.wss?uid=swg21668005.