Re-boiling of tea?

You should try to make up your tea and keep the tea bag / herbs (or whatever you used to make it) in the thermos and reboil it with all the ingredient and just filter for serving. I have never tried but it MAY work, i will be curious if it works or not

Is there a particular reason you do not want to use a Thermos? I'm not quite sure I understand the question, but steeping one cup at a time with water warmed in an electric kettle could solve the problem.

If you'd rather steep one kettle at a time instead of per-cup, steep in a thick ceramic teapot other vessel that is more likely to hold the heat over time.

Adding milk will always cool your tea.

I use one of these guys to make a single cup of hot tea and let it steep just as long as I'd like depending on the leaf at hand: http://www.thinkgeek.com/product/96bb/

Cast-iron teacups hold heat for quite some time. Similarly, a cast-iron teakettle can have the leaves removed and still keep the tea hot. I always use a cast-iron pot when I'm making multiple cups to drink in sequence.

Much of the flavor and aroma of tea comes from volatile oils/compounds.

The heat applied to tea leaves while steeping them is key to releasing those volatile compounds but when you reboil the tea, a large portion the flavor compounds in the water are likely just going to be vaporized. The end result is the reheated tea will have very little 'tea' flavor left. The tannins however are less volatile and remain in the re-heated tea hence the bitterness.

A pre-warmed ceramic teapot would probably due the trick for keeping your tea warm for 5-10 minutes.

This cute problem is centuries old.

Put your teapot on top of a simmering kettle on the stove. The steams from the kettle keep it at the perfect temperture and steep it properly.

You could use a tea cosy for your teapot. Put the tea cosy on the teapot as soon as you're water is boiling, and it will keep the tea warm for the next 10-15 minutes. It won't be boiling, but it doesn't need to be.

Traditionally people would make very strong tea, which would go cool or even cold, and add hot or boiling water to thin it down to drinking strength and heat it up at the same time.
The (Russian) Samovar was used in this tradition, in England they had a small teapot and a much bigger hot water 'pot'.

It does not bring your drink to boiling heat but that is right as nobody can drink boiling tea, and many people even suggest to use water that is no longer boiling (or even never came to the boil) to get the most out of your tea.

If you do not want to go to that method, the best way to use a good thermos is to pre-heat is with hot water, then replace that with boiling water for a few minutes, then poor poor out the boiling water (which is no longer really boiling) and replace it with tea without allowing the thermos to cool at all.
Depending on the quality of the thermos it should keep hot enough for you for a few hours. If yours does not keep hot enough for at least an hour, you should replace it as it is faulty.
You can improve the time a thermos keeps hot by adding an extra layer, like keeping it in a thick layer of towels or in a sleeping bag, making sure that the 'open' end is well covered.