I think the same rules would apply as with paint. If you are going to get back to the job soon, within the next day or so, the easy way is to put the brush in a plastic bag and tie it tightly, and you shold be alright.

If it's going to be longer, stick it in a jar with the bristles entirely submerged in some appropriate solvent - it should say on the tin or bottle what that would be.

If it's water soluble stuff you are using (which it won't be with polyurethane), the thing to do is wash the brush well in warm water, with some soap or washing up liquid, till it's clean. Then it'd do no harm to submerge in a jar full of water, to stop it drying out.