Either get lucky by asking the right person at the right time or get to know someone who does it. Try to get on the local crew at your local venue(s) to make contacts. It's not all glamour, it's bleedin' hard work. Especially if you end up touring with an act whose music you loathe and you get 3 or 4 months of it. You aren't going to see this job advertised in the local paper.
Yes. I used to be a backline roadie for David Essex in my dim distant youth. I got that through a friend of a friend who was stage manager. I do work for the local crew now (amongst other theatrically based work), I'm not sure I'd like to do touring any more.
Groupies and David Essex probably haven't gone together in the same sentence since the late 70's. I wouldn't minded having a go at Girls Aloud when I worked on their gig a month or two back.
In my experience, most people realise that you are doing a job that helps them. I've never come across any nastiness but it happens occasionally so I've been told. Usually by people new to the industry, who recently got a break.