A few things that I think one needs at the flight line with a glow engine:

1 or 2 new glow plugs
glow plug wrench
glow starter and back-up
prop wrench or c wrench
extra rubber bands or nylon bolts (depending on plane)
fuel and fuel pump
extra props (either the size that you are using or the size range for your plane(s)
needle nose pliers
knife of some sort
3 straight and 3 Phillips screw drivers range from extra small to normal size
starter or chicken stick
something to secure the plane at the field, either a rope and a stake (long screwdriver works) or starting stand
tape to patch any small holes

I also have a bin of misc. parts and tools so that I can fix many small things on my planes or my friends planes if needed.

It depends on how organized and prepared you and your planes are, also who is also at the field and if you may use some of their tools if needed.

hope that helps, and just remember that this is my list and what I am comfortable having at the flight line, I also have a small airplane trailer that has some other stuff (extra dremel, etc.) that I bring to the field.

Jon