I've been addicted to Civ 4 for a long time, but I'm having trouble getting into the game lately because it takes so long to keep track of all my units. What are some tips to speed up my gameplay? Keep in mind that the PC is not the factor, I'm not saying the performance is too slow, I'm just saying there's too many units to keep track of towards the end. What can I do to better manage my units so it reduces the time the later turns take?

Play on a smaller map, play a scenario (these often give you smaller empires to deal with), or build fewer units. Automating workers can help, but this can screw up a CE, so be careful or turn on "AI leaves existing improvements". Or, find a way to win before 1850, when the sheer volume of units can start to get ridiculous.

...1850...the sheer volume of units can start to get ridiculous.

You get that problem? Oddly enough, I never have that.

*strokes the button that says "Launch Nuclear Weaponry"*

No I mean, like strategies or something to keep better track of my units? The stack management can get so tedious... I know about all the game settings and all that, I'm thinking of some better way to effectively manage my units? Maybe there's some hotkeys I'm missing or something, I know rally point and that helps some but not enough...

Once you have assembled a stack it's like a single unit. The tedious part is managing the distribution of newly built units. There is not much one can do about it except not waging war in modern times... which I often avoid out of pure laziness.

Which game speed do you play? Unit numbers are much higher on epic speed and horribly high on marathon.

edit: But in case you missed them here are some basic guidelines (though they're so basic that I think you'll know them):
1. Automate unit production in your garrison cities by pressing ALT while clicking on the unit - this unit will now be built indefinitely (you can make alternating indefinite queues by pressing ALT+Shift)
This will save you time because you get fewer pop-ups asking you what to build next.

2. Make a rally point where all newly built units will automatically go to (Select all cities by pressing CTRL and then the name bar of one of your cities, then make a Shift + right click on the tile where you want them to go -> a yellow circle will appear)
This way you don't have to give orders for every new unit. They only ask you for orders once they have reached the rally point. Then you just click the sleep button and they stay silent until you send them on the march into enemy lands.

3. Of course you move your units in stacks and not individually. You can select all units on a tile by ALT+ left mouse button.

4. In the options menu, select Quick Combat and stack attack. Only very noobish players watch the combat animations when attacking or defending in a battle where dozens of units are involved... with stack attack/no animations, even the largest battle lasts only a heartbeat.