I've never done it with bike plastics (but I've done it with other, more trivial things). General consensus is a soldering iron with a nib you don't mind knackering and a load of cable ties for use as "solder" (tightfisted tip: keep old ones you cut off for this purpose- saves melting perfectly good ones!). "Spot weld" the two pieces together in several places (quantity depends on how long the bits being joined are). This is just to hold them still/together while you work on them. This can be done (unless it's very thin plastic) by just melting the two bits together- no need for the solder. Once this is done, check the front looks okay (you might be able to see a bit of light through the crack in places as of course that hasn't been sealed yet) if it's not right, at least it's not to much hassle to desolder the two bits at this stage and try again. If you're happy with it, use the cable ties and melt them so that they fill the crack. Check the front from time to time to make sure none is getting though the gap onto the front (if it's a wide crack). Repeat this along the length of the join till it's all finished. Let it cool, then give it a little flex/put a slight bit of force on it to ensure it's reasonably robust and won't crack as soon as the engine starts vibrating. Then, if you're happy, stick it on the bike.
You can get cable ties in all sorts of colours, so depending on the crack/visibility of the area, it might be worth getting cable ties in the colour of your bike (easy for me- black!).
Needless to say, do all your "welding" on the inside of the panel, so it'll look nicer from the outside.
EDIT: Oh yes and as is always the way with these things- test the soldering iron on it somewhere where it won't notice first. You don't want to find it goes through it like a hot knife through butter/you've got it turned up too high along the crack you're trying to fix.
