Im not telling anyone to take any of the following into serious consideration.
These tips and advice are only my suggestions, no way shape or form am I telling anyone how to run there server.
It's your server you do what you want. I am not trying to be a cocky a-hole. I'm just speaking from experience.
I have tried just about everything, and I will tell you what has worked best for me.
The number one thing to remember is: People hate downloading.
First, look here. You will want to choose about 6-7 maps that are popular (try to evenly shuffle the basic maps). You may choose maps that you like that are not very popular but only 2-3 of them at a time. Totalling your map count to 9-11 maps. Make sure these *rare* maps are under 5 MByte. The smaller the better. If you can find a map that is 2 or 3 MByte then you are doing great. Also when it comes to maps. Playing new maps can be fun, same maps can get old. BUT only change 2-3 maps at a time every 45-60 days.
Second, when it comes to creating custom pk3's. They can be nice, but if your not careful they can be more harmful than helpful. They can create lag and thats just for starters. When I make a custom pk3 to put a server I make sure to incorporate EVERYthing into 1 single pk3. Not seperate pk3's. The more pk3's on the server The Worse off you are. (this includes maps) I'm not going to tell you what to put into your custom pk3 after all thats what makes it "custom" but I will say that to many sounds can be bad because telling the server to play a .wav file can lag everyone on the server. Ecspecially large .wav files. One last thing I will say about custom pk3's is that no way should you make a person download a "soundpack" and not give them any vsay's to play with. Thats just mean LOL. So remember: Keep everything into 1 pk3 download. (If you use custom player skins you should make both teams have the same skins only different colors, that way no team has the advantage.)
Third, when it comes to helping to reduce lag which can be the worst issue with any NQ server that runs on 2.55. The first thing to do is go through all your configs cut back as many configs as possible. With NQ all you really need is the server.cfg which should exec the shrubbot.cfg and noquarter.cfg which in turn exec's the mapcycle.cfg Go thru the server.cfg and the noquarter.cfg (or whatever you have them named) and remove any uneccessary cvars. cvars that are in the cfg with a default # are pointless. Becuz if you remove them they will be default anyways. Removing these uneccessary cvars will help the server to quickly exec the configs and should unltimately reduce lag.
Fourth, omni-bots...ppl hate them, but they can be your friend. Maxbots 12 (meaning there will be 12 bots but 1 bot will leave when 1 perscon connects, 11 real ppl on will mean there will be only 1 omni-bot, and make there skill level around average. Skilled bots are rediculous, and stupid bots are boring, but if you get there reaction time to be somewhat "human-like" they wont be as bad to play with. Also you can fool amatuers to thinking a bot is a real person if you give the bots names that people wouldnt expect a bot to be named. (Don't follow a theme with bot names, people catch on, example: Red Vampire, Purple Zombie, Grey Werewolf) use random bot names such as: Big Bertha, rip-the-cat, n0rthern3r. (Not all bots have to be named with color) Also a good thing to do is to change bot names once a month or so. Always helps peoples mood's to see new names.
I cant garantee anything, but these are a few things I would do.
Those are the main things, other than that, get on and play! be patient!
I might just add a few more random tips here in a "reply" but as for now...It is late, and I am tired.
Good Luck!
Subject: Tips and Advice for Increasing the Average Amount of Players on Your Server 
