Latvia is a Baltic country. There are a lot of hitchhikers and hitchhiking is fairly easy. Even in the middle of nowhere, along important route, even the E67 (Via Baltica), it's quite easy to get a ride.
It is allowed to hitchike in Latvia (at least there are no laws which forbid that), but of course in places where you do not danger road safety. Very, very rarely (but still) people expect you to pay for a ride. It is a common practice here to use signs of the cities, countries where you want to get a ride, but thumb also works fine.