I have a rough idle on a 1996 Toyota corolla DX. I replace the distributor cap, rotor and plugs but it still idles roughly. What should I look for?

Clogged injectors, fuel pressure, weak ignition spark, valve timing, low compression...
Obviously these are things that require a certified shop with proper diagnostic equipment, but with the proper tools and knowledge, it can be performed at home too.
Start by pulling one plug wire at a time to see if engine rpm's change, and the least affected will draw most attention.
A little FYI: The most I see on these are bad coils in the dist.

Clogged injectors, fuel pressure, weak ignition spark, valve timing, low compression....
Obviously these are things that require a certified shop with proper diagnostic equipment, but with the proper tools and knowledge, it can be performed at home too.
Start by pulling one plug wire at a time to see if engine rpm's change, and the least affected will draw most attention.
A little FYI: The most I see on these are bad coils in the dist.

Thanks

I forgot to mention that it is only when I have it in gear that it idles roughly. It accelerates and drives smoothly.

OOOOOOH, that! Simple... it's the engine mount bolted to the passenger side strut area. Toyota calls it an "insulator"
You have to support the engine while changing it out. A block of wood and a floor jack under the oil pan will do the trick.
When describing this problem you should have said 'engine vibration in drive', but most people call it "rough" running. No biggie.

Replacing this will get rid of 90% of it. Replacing the other 2 mounted to the transaxle will make it a new Corolla again.

It idles roughly, when you put it in gear, because you are placing additional strain on the engine. Consider, in addition to what kitch428 recommended, cleaning the Idle Air Control Valve, throttle body, and ports:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-7.html#post1054149

OOOOOOH, that! Simple........it's the engine mount bolted to the passenger side strut area. Toyota calls it an "insulator"
You have to support the engine while changing it out. A block of wood and a floor jack under the oil pan will do the trick.
When discribing this problem you should have said 'engine vibration in drive', but most people call it "rough" running. No biggie.

Replacing this will get rid of 90% of it. Replacing the other 2 mounted to the transaxle will make it a new Corolla again.

Thanks,
You have been a lot of help.