Recording/reporting the event
An important thing to get right is the time of your observation, and even more important the time standard. In the UK, during the summer, we add 1 hour to GMT (or UTC, or 'Z' time...they are all interchangeable for our purposes), and call it British Summer Time. However, international meteorology runs effectively to GMT. So, when you report a phenomenon, use GMT. When BST is in force (in the uk), this means taking one hour off your watch time, so a waterspout seen at 7 pm on the south coast in mid-July should be reported as occurring at 1800 GMT. If you suspect your watch/clock might be in error, check against a time-signal (or similar e.g. teletext clock), and adjust accordingly.
Double-check the date as well! This might not seem an obvious point, but particularly when you are reporting something a day or so later than the event, its easy to get the days mixed up. Watch particularly the time around midnight when we are on BST. Something happening at 15 minutes past midnight by your watch on the 15th, should actually be reported as happening at 2315 GMT on the 14th.
Locations are most important. Its easy to report an exciting event as having happened, and forget to tell us where you are! Have a 'sig file' made up that includes your location, height amsl and other important facts, and use it as appropriate. Obviously if you are out and about, then include as much information about the observing point as possible, or if on a car/train journey, the area where the observation was made. This includes not only the lat/long and/or grid ref. (or town/village if that's easier), but a description of the terrain, location of adjacent water surfaces, direction/height of hill/mountain ranges etc., if they would not be universally known. These latter points of information might help diagnose phenomena in difficult situations, and allow others to relate their observations to yours.
Get into the habit of having a scrap of a pencil and some paper with you to note down important details for later transcription to the newsgroup. Even better of course would be a dictating machine, or a palmtop to record the events. Also, keep a copy of what you post, if your newsreader doesn't already allow this. Someone may want to follow up your report weeks, or even months in the future. Don't rely on your memory...it plays tricks! Always note down the important features ASAP after the event, preferably as it is occurring.
When photographing events, as well as the usual location, time, date etc., note down the readings from the camera...f-stop, film speed/type, shutter speed etc. What state the sun (or other illumination) was in..cloudy/part cloudy/clear; behind/in-front of observer etc. The information may not be needed, but then again, it might! Camcorders are becoming more popular for recording weather events. However, even if you think you have the world's finest footage, make a brief note of the event in longhand just in case what you saw doesn't quite live up to expectations when the tape is re-played. Try to get objects of known dimensions and distance in your shot so that some comparative assessment of the tornado/waterspout etc., can be made. In the specific case of photographs of hailstones, include some form of measure, such as a centimetre rule.
As well as your own report, try to gather other eye-witness accounts, particularly in the case of 'severe' weather events. Newspaper cuttings are invaluable, even if they turn out to be rather sensationalist. If you do actually 'cut' out the report, make sure you annotate with the date of publication, publisher's address etc., in case anyone wants to follow up the news item. Make a note of any local radio and tv reports, and in the latter case try to record the news report when any action shots are broadcast.
See also this question in the FAQ.