Also, if you don't mind some unsolicited tips, it's a good idea to clean the negative carrier first with an antistatic brush or cloth before inserting the negative.
If you've just pulled the negative from it's sleeve / envelope, after inserting it in the carrier, give it a few blasts from a bulb blower (Giottos Rocket Air Blasters are great) before inserting it in the enlarger. Some folks like to brush the negative first, but I find if the negative is already reasonably clean, then it's not necessary. I don't use canned air as I found it was too much pressure and can leave residue/condensation on the film if held too close.
If I see a dust spot on the print, I'll pull out the carrier and give the negative a few more blasts of air without removing it from the carrier. 90% of the time this does the trick. I very rarely have to recompose or refocus after putting the carrier back in the enlarger, but it doesn't hurt to check.