Depends on which bait you are using.
Most will respond to microwaving. Unfortunatly trial and error is the only way to find out for sure. Try ten second bursts untill you find out the optimum cooking time. A good tip is to drill an hole through the boilie first and place a cocktail stick through the hole wilst cooking. This is done because microwaving usually leaves the bait very hard and brittle. Trying to drill the bait after cooking usually results in them splitting. When baits are prepared in this way a lot of the flavour is burned off. Many anglers like to glug their micro popups with a complimentary soak, to help negate this affect.
One company sells an insert kit, I think it may be Gardner could be wrong. Whith this, a tool is used to bore out a cylindrical section out of the centre of the boilie. The portion of boilie that has been removed is replaced by a piece of cylindrical foam. This foam is supplied in various colours to compliment your bait. Cannot comment upon how well this works as I have never had the need to try one.
If all else fails why not just put a cork ball above the boilie, ala snowman style. It does not look pretty, but I have seen no evidence to suggest that it is less effective than any other method. I witnessed a twenty six pound common caught on such a rig. From the north west I should hastily add. Good fish in our neck of the woods.
Couple of points to add.....
when microwaving baits, allow them to cool before testing them or they will just soak up the water and sink.
The foam pop-ups were originally made by Marvic, they sell a boilie punch which pushed out a cylindrical plug of the bait which you replaced with foam.
F** have since 'launched' there own which is obviously a superior product.
I believe Gold Label sell foam discs, which you can place between two halves of a boilie to make a burger effect.
Carl, what I used to do when I did not have popups with me was similar to Andrews suggestion of the cork ball but I used to simply put a bit of rig foam between the bait and the hair stop, no drilling no fuss and the fish did not seem to mind it at all. It does not need much to make it popup and you can also glug the foam in flavour if you want.
Andrew, Rik, anyone....I have a bit of a "mare" with the snowman setup. I want the boyant bait to be on top but I dont want the bottom bait below to slip down the hair....I think Big fish adventure make a sort of snowman plug to stop this but is there a better less fiddly way ?
Pauls right (makes a change) rubber float stop aka DAM with the addition of a sequin if required so the stop doesn't pull into the bait when it swells.
I thought I showed you that when I was over?
Other than that, the snowman plugs are made by <a href"http://www.gardnertackle.co.uk/welcome.htm">Gardner</a>
You could try tying an over hand knot in the hair. Make the length a little shorter than you actually need. If you then thread the boilies with a fine needle the sinker can be forced onto the knot. Simple but effective.
Shelflife baits have already been boiled, so Microwaving them won't make them bouyant. Microwaving, grilling or dry frying really is only effective if you do it to non-boiled baits, instead of boiling them. If you are going to do this you need to keep the baits on the move otherwise you will end up with burnt spots of baits that have cracked or split.
With shelflife baits it is best to either buy the dedicated pop-ups to match them (Nash give you 5 free pop-ups in every bag of shelflife baits) or to use the Boilie Punch as Rik suggested. Without a doubt, the best allround commercial popups available are Nash Airballs, Mistral's PopUps or Ian Russell's Heathrow Baits Pop-Ups. With the Airballs they do not lose any of their bouyancy for at least 36 hours in water even when pierced!!! They are also soft enough to push a Baiting Needle through easily rather than needing drilling.
Microwaving boiled baits will make them float, but as you suggested you have to keep them on the move.
My most buoyant and successful floating hookbait, is basically a boilie mix, which has beem bolied and nuked.
(I'll detail it in my floater article)
Nash airballs are very effective, I think they contain rubber/silicone which makes them so buoyant.