blow air on it to move the dust out of the way and vacuum clean the surroundings to make sure there is minimal amount of dust in the room.

If you can't get all the dust off from the lenses, you can use special cleaning optical paper (you can buy that from almost any optics vendor) to wipe the dirty surfaces.

Another alternative is to use ultra sonic cleaning bath in a mild solvent (alcohol for example).

Which ever way you go about it, make sure the method you choose does not damage the surface (scratches) or anti-reflecting coating! Ask the manufacturer if in doubt!

Agree with Thierry. Just want to add few more details. You can usually buy such cans with a compressed air designed specifically for lens/optics cleaning. If you don't have it, (assuming that it is just dust particles and nothing else) you can use isopropanol or high purity ( important!) ethanol with soft lens cleaning tissues. Fold the tissue several times, hold it with tweezers, add few drops of a solvent and lightly (important) wipe out the surface in a single motion. Don't use the same tissue for more than one wiping. Take another one and repeat it again if necessary. In principle, high purity acetone can be used as well, however for removing dust particles the two solvents mentioned above are more than adequate.

Yes, high purity is really important. I strongly recommend that you need to get high purity more than 98 % at least. Otherwise, you might get a chance to see there is residual oil-like liquid on the optics..It is not good.

You need to start with an air blast to dislodge any large particles. The canned air is generally a bad idea as there is always a risk of propellant mist getting onto the lens. I tend to use the manual bulb versions that work by hand pressure. I prefer Edmund Optics' Air Blower with Compressor, as it features a secondary chamber that boosts output force when you squeeze on the bulb. The part number is #57-607 and it goes for about $20 USD. Once you have managed to get all the large particles blown off, you can use highest grade (purity) isopropanol with proper lens tissues to gently clean the lens. If you can get the lenses apart, drape a tissue over the lens, add a drop of isopropanol, and gently drag the tissue off the lens, being careful not to apply any pressure. Another technique is to use a gentle spiral pattern going from the inside of the lens to the outside. Repeat a few times until clean. 

Oh, and note that Acetone will melt glue and plastics, so I don't recommend it. Isopropanol is much safer.

Thank you all for answers; we used air blower like vacuum pump. That's important, position of collimator must be upside down (lens top). Gravitation is so helpful.

Squeeze the air blower, place inside of collimator and release the air blower.