When it comes to concealing, the name of the game is Neutralizing. Any spots you have on your face that are considered a discoloration, you want to counterbalance the color with the opposite color on the color wheel. Remember my post called "A Solution for Dark Circles"? I will give you the website for the color wheel again here (www.colormatters.com/colortheory.html), but basically the approach is the same for your entire face as it is for your dark circles.
For example, let's say you have some brown discoloration on your face. With which color should you cover it up? Well, if you take a look at the color wheel, the closest color to brown would be the dark orange, and if you look opposite the dark orange, there is light blue. Therefore, you would want to conceal your brown spots with a light blue color. Unfortunately, light blue concealer is not very popular. I have not seen any light blue concealers available anywhere except from www.CoastalScents.com, so you might want to try there first.
For me, the Lise Watier Portfolio concealer has been very helpful, and so has MAC's Studio Finish concealer (shown below, and available at www.MacCosmetics.com for $16.50). However, even though the concealer is supposed to do the brunt of the work when it comes to neutralizing, it is always important to remember that your foundation also plays a key role in concealing your spots. Neutralizing your discolorations means that you have balanced the discoloration so it is not so dark or conspicuous anymore; the work of concealing your concealed spots must be done by your foundation. In other words, it is never enough to just conceal and then walk out the door.
Also, when it comes to liquid foundations, I have found it helpful to conceal first, and then apply liquid foundation because it all blends together better this way. And if you use a powder foundation, I would strongly discourage concealing with anything cakey. MAC's Studio Finish concealer is great with liquid foundation, but under a powder foundation, its emollient-based moisture content would show through as a dark spot on your otherwise lighter-colored face. Trust me, you don't want to go out with this look. Instead, use a concealer that dries completely, and then apply your powder foundation. An example of this type of concealer might be Maybelline's Instant Age Rewind concealer, available at most drug stores (I don't know the price, sorry).
Remember, conceal with the opposite color on the color wheel, and never forget to do your foundation. I hope this helps, and please let me know if you have any questions or comments.