I've been running now for the past few years, and run a 5k or 10k road race every couple of months. I've always wondered what is the best way to pin on a racing number, and where the best place to pin it to is. - Feel Like a Number
This is an excellent question, and typical of the kind of question that this little section of Run The Planet is prone to answer.
Before I pass on any wisdom concerning the correct way that you should affix your race number to your T-shirt, let me list for you the ways in which you must never do so.
First: never pin your race number directly to your body. I understand that some runners like to run shirtless (less so for most women I suppose), but one has to remember that your chest epidermis, or "skin", is in fact a part of your body, and that a non-anesthetic application of any pointy object is going to result in undesirable discomfort.
In the same sense, you should avoid using an epoxy, super glue, hot wax or staples to affix the number to your skin. Runners are encouraged not to have their race numbers tattooed to their bodies due partly to the expense, pain of application and permanence.
Some races give specific instruction as to where race numbers should be pinned. Shirt fronts are common, although some race directors will allow numbers to be pinned to the front of shorts or even the back of shirts.
The International Association of Athletics Federations (IAAF) rule 143 for general competition, parts 7 and 8 read:
7. Every competitor shall be provided with two number bibs which, during the competition, shall be worn visibly on the breast and back, except in the pole vault and high jump events where one number bib may be worn on the breast or back only. The number bib shall correspond with the number allocated to the competitor in the programme. If tracksuits are worn during the competition, number bibs shall be work on the tracksuit in a similar manner.But most road races require only that the race number be pinned to the front of the shirt so that it's most visible by race officials and easier to pull a tag on the bib off at the end of the race.
8. These number bibs must be work as issued and may not be cut, folded or obscured in any way. In long distance events these number bibs may be perforated to assist the circulation of air, but the perforation must not be made on any of the letter or numerals, which appears on them.
One thing is clear: never pin your number to your "rear-end"! This will almost certainly cause unwanted distractions to those runners you pass by and race officials are probably less inclined to focus on the posterior section of your body.
The best way to pin your bib number to your shirt is to do so slowly and carefully, keeping the number side facing out for all to see. The best place to do this is probably in a private, quiet place sometime prior to the beginning of the race!
One last point is that I always use four pins, rather than the two that many race officials will hand out. I like to pin my number at all four corners of the bib. I've run races where my number actually fell off in the heat of the race! You don't want your number to come off while running, it's not a good feeling to know that you just run your best race and won't receive an official time because of a missing paper bib!