You do have to be able to get up there somehow, of course. Cherry pickers are used for decorating tall trees in town squares.

My 75 year old neighbor did this last year to light up his 100 foot pine and I think its pretty crafty thingking. He attached the lights to a arrow and shot the arrow up through the tree until he got enough through that he was happy...
25' isn't to high so you could shoot it right over the top and string the lights fairly easy - in theory anyhoo... LOL

I realize I'm a bit late here, but I still haven't hung lights on my 25' spruce. I use a long bamboo pole attached to an old broom handle. On the end I made a fork out of two pieces of bamboo that is wide enough to catch the wire easily but narrow enough that the lights don't slip through. If you email me I'll gladly send you my address so you can send your monetary thanks for keeping your limbs unbroke and your holiday trappings frustration to a minimum. I'm quite certain the plan will work so well that generations from now your offspring, with their 100 foot light-hanging poles will still be sending my offspring money and gifts in gratitude. And since I accept responsibility for both the good and the bad, metal mop handles should not be substituted for old wooden broom handles (ya gotta have the lights on to see how your'e doing) and goggles should be worn when installing and viewing the light display.

I'll throw in this free tip, to ensure return business: In an area out of sight (under the tree) drive a stake into the ground. Tape your extension cord to the stake with the business end up. tape your connections between extension cord and light plugs. Cover the stake and connections with a plastic bucket. This will keep the stuff out of the snow and wet.