Install Lion on new SSD on MBP 2009
As per Patrick's answer (above).

However, for all those people that would like to carry across all of their data and apps AFTER doing a clean Lion install, you could do the following :


take a snapshot of your existing drive using either Carbon Copy Cloner or SuperDuper.
make sure your external drive that contains your "snapshot" is plugged in and then reboot your mac
during the boot process, hold the alt/option key down when you hear the "bong" noise ..
select the external drive and confirm that you can boot using the external drive's "snapshot", in case you need to get back to a working boot situation.
at this point (while you are still booted from your external drive), you can now fire up "Disk Utility" and delete and then re-create a blank partition on your internal drive.
now re-boot and use the bootable USB Lion drive (as per Patrick's link in Patrick's answer, above).
Run through the install of a clean Lion OSX installation.
Once rebooted into a clean Lion install, now plug in your external drive that contains your "snapshot" of your Snow Leopard (or Leopard, or Tiger) that you made at step 1.
run the Lion "Migration Assistant", and tick what user accounts, and data and applications you want to carry across to your clean Lion install.
Enjoy a clean Lion OSX installation with all of your mouse preferences, desktop backgrounds, applications, and user data carried across.


(note, this does not make your clean Lion install "dirty", since all of your old APE hacks etc are NOT carried across via "migration assistant").