We have seven layers (lines) of PBS. If PBS gets to the 7th layer and can't accomodate your requests, you'll get screwed with whatever PBS wants to give you. I use lines 1 and 2 for my "dream schedule." I build the exact schedule I want in the first and second layer. If you bid pairings, don't put any other preferences in that layer, it might prevent you from getting that.

Heres how I bid...
Layer:
1 - Specific pairing and days off
2- Specific pairing and days off
3 - Locals, show/release by xx:xx, credit > xx:xx
4 - 2 days, show/release by xx:xx, credit > xx:xx
5 - Locals
6 - 2 Days
7 - Three days

I'm senior, so I usually get awarded my first or second choice, but if youre junior, you can't be picky. Dont put too many properties in your layers. The more you have, the higher your risk of screwing up your award. Get less restrictive the farther you get into your layers.

It's only as hard as you want to make it. Some folks spend about 5 minutes on their bid, some take the whole 7 days...