Signs show that you're not a fresh escape convict from county jail and that you've planned your trip and have a destination. The brighter you make your signs the better, but don't make it so they can't be read from a distance. Step back from your sign and pretend you were the driver passing by. Hold the sign in one hand and stick up your thumb with the other. Don't be lazy and stick the sign on the ground where no one will read it.
Signs can also be a bad thing.
If someone sees your sign and is not going to where your sign says, it may prevent them from stopping. Use your own judgement, experiment and see for yourself what works. People driving by will look at your face to see who you are so remember to try and smile or at least look happy or friendly. Before getting into a car, be sure to make eye contact and have small talk with the person driving. Also try and take extra precautions or avoid getting into a car with more people than you.