Tire plug kits today are much better than they were a few years back. The chemical and mechanical bond between the plug and tire compound is what is critical. If you have a nail/screw in your tire you should get it out at soon as possible (more to come on that) and get the repair done immediately thereafter--remember to cut/remove any steel wire from the chord, if it is sticking through as it could cause further damage.
Endurance rider--those who run X-cross country/world have to patch their tires in the oddest of places. Places where getting a new tire is out of the question and it would increase their costs dramatically. Follow manufacturers recommendations.
Why get the object out ASAP?
Soon after you plug the tire you need to get it to shop to have the tire removed and inspected from the inside. The nail/screw (I once had a spoke from a Jaguar rim jammed through my truck tire) can do severe damage to the inside of the tire as it moves around inside. It can score the sidewalls something fierce. The side walls take the most torsional abuse and are most susceptible to failure from such damage. If the internals are at all worn---REPLACE THE TIRE. If not, you can most likely get away with a patch/plug.
There are different opinions on the shape, location of how to install a patch. Again, follow manufacturer directions, or ask to see the same if someone else does it for you.
Finally, if you don't have confidence in repairing the tire, replace it. The last thing you want is to start thinking to yourself, "Gee is my tire gonna hold in this turn or is it gonna blow out and I'm gonna crash?" Just as you're going into the twisties in the sticks. You need to focus on what you are doing--riding with proper technique.
For further information:
http://www.motorcyclecruiser.com/tec...air/index.html
~P