If your parking sensors are in the same place as mine, ie on the outer corners of the bumper, you probably wont need to take the bumper off to get to them.
Even if they are anywhere else, you most likely wont need to take the bumper off either.
if they are on the corners, then just remove the nearest rear light cluster by popping out the two round trim pieces next to the lights and taking out the screws under them. The clusters should then come off with a bit of twisting and pulling, also if you look in the boot under the carpet trim behind the lights there are two square push in holders that you may have to compress the lugs on, if they dont pop out.
After the lights are out, you should be able to stick your hand into the bumper and get to the sensor, on mine the wires run through the cluster wiring grommet, so you would need to take the lights out anyway.
If they are away from the corners, and you cant reach them using the above method, then look under the car at the front edge of the bumper, there should be a long black plastic trim, with five retaining clips in it. On some cars these may be screws, but on mine they were the two piece clips which you remove by pulling out the core that looks like a screw head. The rest of the retainer is part of the plastic trim.
When you remove all five cores, pull or prise out the plastic trim. This may cause a metal part the same length as the plastic trim to fall off, if not then pull it off, it will only fall off anyway.
Now you can pull on the bottom of the plastic part of the bumper (not TOO hard!), and you may just be able to do the job this way.
When you come to put it all back together again, the metal part goes behind the bumper, sandwiching the bumper with the plastic trim.
There are also three metal lugs on the bodywork that the retainers have to go through, be careful of these as they bend easily when you are trying to push the retainers through.
The light clusters should come out leaving the two square retainers in place with two pegs that locate in them on the cluster, if the square retainers came out then just make sure that they both go back in again properly with the lugs spreading either side, if they come loose after you have taken the clusters out then try not to drop them down into the bumper cavity, especially if its getting dark, and you have nearly finished the job, and the batteries in your torch are nearly flat.
If you have got the same corner sensors as me, however, I would recommend that you dont bother, as these things are useless at best.
The best thing to do in this case is to fit an after market set of sensors, I have just finished fitting a set on mine consisting of two sensors at the front and four at the rear.
The ones at the rear were a doddle to fit.
The ones at the front however, made me shout rude words.
A lot.
However they have been well worth it, my feeling is that it is a lot less stressful, and a lot cheaper, to fit these things than to have to replace or repair the bumpers on your car.
you may want to visit here:
http://www.lexls.com/tutorials/body/trailerhitch.html
For a few more pointers.