Air leaks can be a difficult thing to spot and hard to avoid when doing major work to the engine's induction system. You engine is essentially a big vacuum pump, and it's always trying to pull air in from wherever it can. Manifold removal and replacement means opening the door to numerous opportunities for leaks. While tracking them down and fixing them is always an option, the best solution is prevention.
High Idle Problems
The basic problem here is one of airflow -- too much of it. An engine can't run without air; it needs the oxygen to burn fuel going into the cylinders. Simply dumping more fuel into the engine will cause it to run rich, unless the engine is very cold and needs the extra fuel. Without some sort of excess airflow, your car's computer -- if it's fuel injected -- would compensate by reducing the amount of fuel injected to bring air/fuel ratio back in line. So, you have to have extra air going into the engine; diagnosing the problem is just a matter of finding the leak.
Manifold Gasket Leak
This is the most likely source of air leaks, particularly if you just replaced the manifold. Intake manifold gaskets have a nasty habit of leaking where they meet the cylinder head, especially when using a metal valley-pan-style metal gasket and aluminum intake. The cylinder head, metal gasket and aluminum intake all expand at different rates, and any irregularity in the gasket or torquing procedure will cause gasket distortion and air leaks. Reusable, rubber O-ring gaskets aren't always quite as reusable as they appear to be, which is why it's important that you use a new one when servicing the intake. The same holds true for carburetor and throttle body gaskets as well as the duct from your airbox to throttle body.
Vacuum Leaks and Throttle Cable
Vacuum leaks are an ever-present concern when dealing with anything on the intake, and they're insidious in their subtlety. Rubber vacuum lines harden over time and mold themselves around the fittings to which they're attached. This happens pretty quickly -- usually within about five years, depending upon the lines and under-hood temperatures. If you have the slack, it's always a good idea to snip off at least 1/4-inch of vacuum line tip when reinstalling it. A vacuum line should require a bit of force to push onto the fitting; if it just slides on like it's greased, then the line has hardened and loosened.
Throttle Cable and Sensor, IAC Valve
This is an easy mistake to make. Throttle cables can easily go out of adjustment when removing the carburetor, throttle body and intake manifold, which is why readjusting the throttle linkage and transmission kickdown linkage is always a good idea after servicing the intake manifold. Throttle position sensors aren't likely to go out of adjustment during this procedure, but checking the position is advisable. Most people will clean the throttle body and idle air control channels while replacing the intake manifold. Some computers are programmed to compensate for clogging in the IAC by holding it open a bit farther; make sure to reset your computer and clear any codes after replacing the intake.
Finding the Leak
There's an old mechanic's trick to finding elusive vacuum and air leaks, and it requires little more than a can of commercially available starting fluid. Starting fluid -- also known as ether -- is a highly combustible fuel used as a supplemental fuel to help the primary fuel ignite. To find a vacuum or gasket leak, spray the suspect area with a short, one-second burst about 3 inches from the vacuum fitting or mating surface. If there's a leak present, it'll suck the ether into your engine. This extra fuel will work with the extra air from the leak to cause a momentary rise in rpm. Just keep the volatile ether sprays short and away from hot exhaust manifold parts, electronic equipment and the alternator.