A specific reason for poor digestion should be sought. Some possibilities to consider:
Poor liver function such that the liver is not providing enough acid and other digestive components to properly digest food. Some possible remedies are to clean the liver, use a multivitamin to provide sufficient B vitamins and other nutrients for the liver to function properly, remove possible infection by bacteria, virus, or parasites if they are thought to be a problem.
Insufficient production of enzymes by the pancreas. Ensure adequate consumption of vitamins, minerals, and other nutrients as well as avoid chemicals which are potentially toxic to the pancreas such as wood alcohols (in aspartame as well as other chemicals). A most important mineral is magnesium, and it is hard to obtain 100% RDA in food, since it is necessary to produce enzymes in the body, which often makes using a 1:1 ratio calcium magnesium supplement beneficial (see the minerals section for descriptions of recommended multiminerals such as the widely available Now Full Spectrum Mineral Caps.
Unfavorable balance of flora in the intestines. Besides the direct effect of causing digestive dysfunction, having a healthy flora is also important to keep the liver functioning properly since beneficial bacteria produce B vitamins which are required by the liver. Although wide spectrum probiotic supplements like PB-8 and Jarrodophilus may be helpful in the short term, eating fresh fruit and vegetables are the best way to stimulate long term health of beneficial bacteria in the body. The best all around probiotic and digestion supplement for the money is a DIY product: cultured cabbage juice.
Following Eat Right 4 Your Type can be very helpful in many cases, avoiding foods which are bad for one's blood type. Note that often, as predicted by ER4YT, blood type Os have digestion problems from eating too many grains and blood type As have digestion problems from eating too much meat. It may also be helpful to follow the tenets of food combining for a few weeks. This can lead to an immediate improvement if there is immediate digestive discomfort from eating.
One of the most popular short term palliative measures for digestive problems is using enzyme supplements before meals. One of the most powerful for the money is Enzymatic Therapy Mega-Zyme. Another is using deglycyrrhizinated licorice (DGL), licorice extract which provides immediate relief for some digestion problems. Ginger, fennel seeds, and aloe (non laxative type) are often used to immediately quell digestion problems. Slippery elm, taken in large amounts (such as 4 capsules or more) has a soothing effect on the stomach and provide gentle cleansing action of the intestines. Turmeric is used with fennel seeds to add antibacterial action and improve the flora, although turmeric itself does not provide immediate action to sooth the stomach and too much at one time can cause burning. Other regimens to consider include those for IBS and Crohn's and GERD.