Like the other Baltics Lithuania is a nice country to hitchhike. There are many active hitchhikers. But when going from Latvia to Poland try to get a direct hitch to the Polish border - it's rather hard getting a car on this highway - it's built for quickly crossing Lithuania!
Like many countries in Eastern Europe hitchhiking on the highways isn't really forbidden. Often there are bus stops where many hitchhikers try to get rides. Don't be scared to walk next to a highway, especially if it's next to a city. The locals and even schoolkids do it, so drivers are used to it.
Get yourself a SIM card if you want to stay in touch with Lithuanians. The cheapest start at the equivalent of 30 eurocents at most kiosks.