You can toss it on the previous yeast cake and can end up with good results after doing so, but there is a lot of other stuff in that yeast cake besides yeast and can cause off flavors. It also often times is done by starting your new batch in a fermenter that has not been cleaned and sanitizied from your previous batch, which can end up causing very undesirable results.
A better method of reusing yeast is to follow the "harvest and wash" method. Its fairly simple, and will give you consistent results. It will also allow you to store the yeast for later use.
1) Make some clean, microbe-free water. Easiest method is to boil water, put it in a jar, then let it cool. If you have a sub-micron water filter made for removing all contaminants including bacteria, then that can be used instead. Also thouroughly sanitize some empty jars.
2) Pour some of that clean water into the fermenter onto the yeast cake. Give it a swirl to make it into a slurry. Wipe the lip of the fermenter with some alcohol to ensure there are no contaminants on the lip, then pour the slurry it into some jars.
3) Allow the jars to sit on the counter for approximately 30-40 minutes. You will notice some heavier sediment settling to the bottom of the jars, and the bulk of the liquid remaining opaque. The heavy sediment that settles quickly is primarily fruit/beer sediment, dead yeast, and other undesirable stuff. Live yeast will still remain suspended in the liquid at this point.
4) Carefully pour (decant) the opaque liquid off of the sediment into another set of clean jars.
5) Set the jars with the liquid into the refrigerator for at least a few days. The stuff that was making the liquid opaque will settle to the bottom, with clear liquid above it. Carefully pour off the clear liquid. The sediment that you are now left with should be white or off-white.
6) You now have a washed yeast slurry that can be pitched directly into a new batch.
I have found that this slurry will also keep very well in the refrigerator, and have successfully used yeast that was harvested in this manner up to 9 months later with great results. If you are planning on storing it, then leave a small amount of the clear liquid on top of the final sediment during storage. Also, when using some that has been stored for a while, be sure to inspect it before pitching it to ensure that you didn't accidentally end up with mold on your yeast. If you were careful about keeping everything clean and sanitized during the process, then there shouldn't be any.
I always recommend using Yeast Nutrient (I use Organic Non-DAP nutrient, Fermaid-O), though that is up to the brewer. Use of pectic enzyme is also up to the brewer. Weather you are using fresh yeast or reusing old yeast will not factor in to the decision of weather to use Nutrient or Enzyme.