Any question answered...

Thumb up high, hint of a bollock in the headlights.

Keep a clean look about yourself, walking is better than standing, signage may or may not help, don't wear a trench coat, looking like a female will get you more rides, don't carry a weapon because someone meaning to do you harm will be be better at it than you.
Mostly, nowadays you're not likely to get any rides.

A sign helps. Internationally, remember to use the local spelling.
Feed roads are better because more drivers are going to or passing through your destination.

I really hope it does make a comeback in the UK. Just before I left I hitched a ride in Gypsy Hill because I'd missed the bus to a friend's birthday party in Thornton Heath. Stuck my thumb out and got the ride in about 5 mins.

I found it much easier in most of EU countries I visited. Scandinavia was tricky at times.

Is there no app equivalent these days? In Europe there is one called blablacar for ride sharing although there is a cost sharing element.

It's been 30 years since I did it.

When you pick your spot make sure drivers have a good place to stop once they've decided to pick you up and try to have your thumb out at a point where they are not pedal to the metal.

If you want to go long distances be prepared for some epic detours.

I never used a sign but maybe it helps.

You pretty much always end up walking until you find a spot, sometimes it's not the best spot but it used to be pretty easy to get picked up within a few hours from even the worst spots.

I don't think it's dangerous but coaches got so cheap it stopped being worth it for longer trips.