There are two things that can cause bumps in handspun:
variability in drafting, the fiber being spun, and twist getting into the drafting triangle.
OK, so there are three things that can cause bumps in handspun. And I am sure there are more, too.
If the amount of fiber in your drafting triangle varies from draft to draft, the diameter of your resulting yarn will also vary. The "drafting triangle" is the little triangle of fiber that arises from your pulling fibers out of the roving. As you pull (attenuate) fibers out of the big, thick roving (even pencil roving gets a little triangle), a triangle is formed from the roving to the drafted-ready-to-spin-yarn.
All spinners do this -- it's not a spinning trick or technique, it just happens as you draft. Make sure the amount of fiber in your drafting triangle stays consistent, and your yarn will be more consistent.
Which brings me to the third point (yes, I skipped #2). If you let any twist at all reach that drafting triangle, it will be harder to draft fiber out of the roving and out of the triangle, and likely your yarn will be inconsistent. Make it easy on yourself -- keep all the twist out of the drafting triangle. Never let it get back there -- pinch it off with one of your hands. If one hand needs to be busy, move the other one up to pinch the twist before the drafting triangle.
Returning to that pesky point number 2 -- if your fiber has texture in it, from silk noil, nubs of other fibers blended into the wool, yarn scraps -- then your resulting yarn will be bumpy. Rejoice! such fiber is supposed to spin into a textured yarn. If you want a smooth yarn, you could work really hard with a fiber like this and get smooth yarn (see the lovely skein there? it's smooth singles from such a fiber -- I've mastered the blend! yay!). But for the most part, such blends were made to be spun into textured yarns.
Now, some texture is unintentional -- VM (vegetable matter, hay, grass and the like) can create bumps. For the unintended lumps, you need to learn to pick them out as you spin. Wool comes from sheep. Sheep live outdoors. Even with scouring, dyeing, blending, and carding some small pieces may yet live in the roving. Keep an eye out for VM, and pick it out as you go. Commercial (Louet, Ashland Bay and the like) combed top is not likely to have VM in it; but small farm roving, processed by the smaller-run mills, will have the occasional piece.
One last tip ... you may also try running your fingers over the length as the final twist is going into it -- this helps smooth it out and twist in some of the loose ends that may be hanging out. It also compresses the yarn, so if you are aiming for a light, airy yarn -- don't do this, as you'll be squeezing out the airiness at the same time.
(based on a post by me to spindlers, 12/12/2001)