Horizontal beams have a hard time picking up, and if they do, they tend not to hold as long. I'd say at least a 10 degree angle, up or down, is good.
Loose, dusty, black liquor is much easier to pick up for lower power. Solid/cooked on black liquor can work well (possibly better) for powers upwards of 300mW (violet).
The speed at which the black liquor sample moves through the focal point affects how likely you are to get a particle. The optimum speed may depend on power and trap geometry.
The direction the black liquor sample moves through the focal point makes a difference. I have had good luck with moving away from the light source and down through the beam (if beam is angled down) or up through the beam (if beam is angled up). You want the surface to be moving such that when particles are removed from it by the light, the surface will not push them out of their place in the beam.
If you use a plastic instrument (like a plastic spoon) to pick up particles from, you may have difficulty due to electrostatic forces between the instrument and the particles in the beam. I have had an easier time picking up from a metal spoon.