Easy.  Just use clingfilm.  If you want it extra smooth, you could use a few pieces and overlap them.  One over the nose, others laid above, below and side.  Then just build up your paper layers.  When it is dry, lift it off, pull off the clingfilm, allow a bit extra drying time and Hey Presto!!
DavidO

I think David's suggestion is the best.  Masking tape is rather sensitive to many liquids, and any oil might be one of the worst.  I would hate to see you ruin all your hard work.

If you plan to make several masks, your original mask base might soon be ruined by water seeping under the cling film, so you might want to consider making a plaster duplicate so you could make many masks from it.

You will need:  enough dry plaster to fill the PM face, a flexible plastic bowl for mixing it in (or a rigid one you intend to discard), clean room-temperature water, an old spoon, and more cling film.

Cover your original form with cling film, then make a PM mask form for a mold (make sure you close over the eyes and nostrils).  Once the PM form is removed from the form and dried thoroughly, paint the inside of the mask form with shellac (probably two coats, letting dry between coats).  This will make it more rigid and help to protect it against moisture -- if it gets wet, it will soften and become distorted.

When thoroughly dry, lay the mask form face down in something soft like a large bowl or tub of dry sand, cat litter or other loose material that will 'adopt' the shape of the face and support it so it won't become distorted when the weight of the wet plaster is in it.  Place a large sheet of the cling film inside the mask, over the dry shellac, and press it into the mask form.  Don't expect it to fit well, it will be loose, but try to work it into the lowest spots --- you don't want a short nose.

Mix some plaster (about enough to fill half the face -- just estimate) and slowly pour about one cup (.25 litre) of it into the 'bowl' of the face, into the cling film.  Use a rounded stick or handle of an old spoon to gently  press the film into the nose and other low spots, to make sure the plaster is flowing into all the low spots (nose, cheeks, chin, etc). 

Then add the rest of the plaster.  Immediately start spreading the plaster up the sides of the 'bowl' of the face with the back of the spoon, up to the edge of the mask.  If the plaster is fairly liquid, it will flow down again, but it will start to set up very soon, so just keep drawing it up the sides of the face.  It will thicken quickly once it begins, so try to make a thick, even layer all around the inside of the PM form.  The inside will be fairly rough, but it doesn't matter.

Let the plaster harden in place.  Leave it in the bowl, in the sand, and don't pick at it.  The next day, or later in the same day, notice how thick the wall of plaster is.  You want it about one inch thick at least (25mm).  If it doesn't look thick enough, make up more plaster and fill in more of the cavity.  You could fill the ENTIRE cavity if you want, but it will take longer for the complete piece to cure and dry, and will be heavier.

When all the plaster work has been completed and set, feel the plaster.  If it's cool and clammy, it still needs to sit and cure.  When it's room temperature, you can continue.

Lift the whole face out of the sand and turn it over into a table, face upward.  Carefully remove the PM form and the cling film and look at your casting.  Does it have an entire nose, right down to the tip?  Are the cheeks filled in?  The chin?  If there seem to be gaps, you could mix a little bit of plaster in a cup, brush a bit of water onto the area where you'll apply it, let your mix start to get thick, then apply it to the areas that need filling out.  Don't worry about perfection.  Just get enough in place.  When it's dry and cured, you can gently sand off the excess.

Once the plaster form looks good, apply several layers of shellac.  Five coats on the front and five coats on the back is not too many.  You want the entire plaster form completely coated with shellac.  Let dry between each layer.

Once the last coat of shellac has dried, you're ready to create as many masks as you can think of.

WARNINGS:  Thick coatings of plaster WILL BURN SKIN!  Thin coats shouldn't cause any problems.

NEVER, EVER put plaster, wet or dry, down a sink drain.  PLASTER WILL CURE UNDER WATER.  And it will ruin your plumbing.

Keep plaster away from your eyes -- it is very alkaline and it will burn them.  Plaster and hair do not mix well -- keep your hair tied back.

sounds good...now i just have to talk my fiance into buying me some...lol i'm broke.