After you have got ar ride the best thing to do, if you are going to Poland, is get out at roundabout just after Kekava (even if the car can take you to Iecava or Bauska). There you will see a lot of traffic going straight to Poland or even further. Just make sure you have a sign.
Few euros option - from the central bus station, take a bus to Kekava, from there find the main road E67/A7
If you are next to RIX airport, try asking drivers of cars with Lithuanian number plates LT