Make sure that they are neither bone-dry nor dripping wet, either of which interferes with getting a proper bond. Damp is good. Recall (or learn) that mortar and concrete do not, as commonly misnomered, "dry" to set, they hydrate to cure, in a chemical reaction where crystals grow throughout the mass, and that needs water to work. A common problem with casual brickwork is excessively dry bricks that suck the water out of the mortar and prevent it from curing properly.
There is also a matter of technique to get mortar to stick to a vertical surface - it wants to be thrown or slapped on and spread out, not applied in a tentative manner.