I have found the Pomodoro technique quite useful. Once I get off my ass to actually start the 25 minute work interval, the conditioning kicks in and concentration appears.
Surprisingly, the first part (just starting) is harder than following through.
It's a valid concern. A single 25 mins block might indeed be too short for a big task but in Pomodoro it's series of such blocks interspersed with 5 mins breaks.
YMMV, but I've found out that the 5 mins breaks do not destroy my "state", as long as I don't start anything that tends to suck in like playing games or browsing the Web. Fetching coffee, going to the toilet, etc. is OK :)
Anyway, the hardest thing is usually starting the first block.
Surprisingly, the first part (just starting) is harder than following through.