If you're not certain that your server can support this, don't try to do it. It increases CPU + bandwidth usage, so only do this if you know what the hell you're doing with your server.
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tf2 hit registration seems to improve at higher tickrates so here's a plugin that lets you change tickrate
https://github.com/daemon32/tickrate_enabler/releases
i think it was originally developed for css, but it also works in tf2.
it unlocks the -tickrate launch parameter, allowing your server to simulate the game at a higher degree of accuracy. it was locked back in 2010 due to negative effects at higher tickrates - such as doors not opening as quickly, and potential differences in how the weapons & game physics handled. not sure how many of these remain in the game today, however
i've done some testing with dozens of people, and found that running a server at a tickrate of 100 or greater appears to have a significant positive effect on hit registration, as well as making explosive jumping feel more consistent. there are effects on air acceleration and potentially other game mechanics as well - i'd like to see testing underway for 6s pugs to see how it feels.
one thing you need to understand is that you can't just set your tickrate to 128 and be done - there are some networking cvars you need to adjust to accompany it, since the default ones are set up for 66 tick.
i've written up a pastebin you can find here, containing a small guide for server owners:
http://pastebin.com/8jDPMWqf
this should help you with finding the settings that are suitable for your server.
thanks for reading, and happy fragging
https://dl.dropboxusercontent.com/u/7539801/emotes/dwayne.gif
if you're a server owner and you've found something wrong with any part of this post, please don't hesitate to contact me so i can fix it - all of this information is from my own testing! i aim to benefit the tf2 competitive community at heart, and i don't want any of this to be incorrect or misleading.