Answer is pretty simple, if you just give couple of minor details:
Which operating system you are using, Windows, Mac or Linux?
Which version of Java you are using, 6 or 7?
Which version of Java you are using, 32 or 64 bits?
How much memory did you allocate in the Launcher options? (cogwheel in right upper corner)
What does your log file say in %appdata%.techniclogs?
What does your log file say in %appdata%.technicbigdig? (ForgeModLoader-client-0.log)