West towards Fishguard
Head towards the entrance to the A40. Right after the roundabout there is a small area on the left where cars can pull off. I would recommend standing a bit ahead of the pull off so you allow cars some time to see your sign and stop.
A bit down the road is a gas station. During the off season months, it seems most people are just turning back to Haverfordwest, but during the peak season more people might stop here before catching the ferry from Fishguard.
North towards Snowdonia
I would recommend hitching to Fishguard first and then trying to hitch on the A487 from there. My sign to Aberyswyth got me nowhere and it wasn't until I flagged Fishguard did I get a ride very quickly. A local hitchhiker informed me that most traffic north of Fishguard will most likely be very short trips between towns.
Other useful info
You can grab a West Wales Rover ticket for £7.50 (2017 prices).