Leaving Boston Edit
Eventually, you probably want to get on to I-90 (the Mass Pike). This goes right into downtown Boston, so find a busy onramp downtown. If you're in the suburbs already, a tempting spot to start might be by the Riverside MBTA station, but you'll probably find very little traffic going your way. If you can get a short ride just as far as one of the service plazas, you'll likely have much better luck.
Route 9 is an excellent road to hitchike on: lots of traffic, plenty of stoplights and gas stations, and a decent average speed. You can probably get to Worcester faster along Route 9 than by taking the Mass Pike.
Around Boston Edit
Buses are cheap (90 cents, including a transfer). Subways (the T) area just $1.25 and likely faster (Some distant stops cost more, as does the commuter rail). Use these to get yourself to a good hitchhiking spot or to your final destination once you're close.