The dangers of hitch-hiking are obvious. So obvious they don't need restating here. And, paradoxically, so obvious they are probably smaller than they seem. There's no statistical evidence, but the added dangers of hitch-hiking are probably slight in relation to the dangers already inherent in travelling by road.
There was a time in the 1980s and earlier when hitch-hiking was a viable means of travelling the length and breadth of the UK, and Scotland in particular, with ease and speed. But times change, and what we might have done ourselves we'd not now recommend to others.
But while we'd no longer recommend hitch-hiking as a means of touring Scotland, there are times when it's a useful option to have. Perhaps bad weather has caused you to descend on the opposite side of a range of mountains from your car. Perhaps a long ridge walk like the South Glen Shiel Ridge leaves you a choice at the end of the day of walking several miles along a busy and dangerous road or hitching. Perhaps you find yourself on an island or a remote part of the mainland without either a car or a Postbus or other means of public transport. Or perhaps you want to extend the range of accommodation available when undertaking a long distance walk; hitching to a settlement at the end of the day, then back again at the start of the next.
And if you are going to hitch-hike, your chances of getting to where you want to be are better if you do it right. The principles are largely common sense. Most important is location. You need to find somewhere where approaching drivers can see you far enough ahead to decide whether they want to stop to pick you up. And you need to find somewhere where drivers will feel they can safely stop. So standing at the "upstream" end of a layby is ideal.
Once you've found your spot, stand there with a thumb steadily extended and try to catch the eyes of oncoming drivers. A couple of "don'ts". Don't show frustration if someone doesn't stop, just focus on the next vehicle. And don't try to hitch-hike while walking along a road. This will inevitably place your back to the traffic, it will probably put you in just the wrong place when someone comes along who might want to stop for you, and it suggests to drivers that if you can walk you don't need a lift anyway.
Another thought. Bear in mind how you look to the approaching driver. You are far more likely to be picked up if the driver has some idea of why you need a lift. So hitching with luggage will often succeed where hitching without will not. Also consider whether you would want someone looking like you in your car. If you've just come off a mountain you won't be able to control your appearance: and you probably shouldn't try. Climbers and walkers will pick up people who look like climbers and walkers. But packing away the muddy gaiters and the walking poles is a good idea. Heavy rain is a double edged sword: a sopping wet passenger might deter some drivers, but you'll get the sympathy vote from others.
On the whole males should try to hitch alone, unless in an obvious walking or climbing group. Females should try to hitch in pairs: again for reasons that are too obvious to dwell on.
So, if you find you do need to hitch-hike, pick your spot, stick out your thumb, and prepare to be surprised by how quickly someone stops to take you on your way.