In some studies Vitamin D has been shown to increase magnesium absorption.
Minerals are generally known to have a competitive effect when it comes to absorption so a high intake of the following could potentially reduce magnesium bioavailability:
Calcium, Phosphorous, Iron, Copper, Manganese
From the National Institutes of Health:
Magnesium supplements are available in a variety of forms, including magnesium oxide, citrate, and chloride [2,3]. The Supplement Facts panel on a dietary supplement label declares the amount of elemental magnesium in the product, not the weight of the entire magnesium-containing compound.
Absorption of magnesium from different kinds of magnesium supplements varies. Forms of magnesium that dissolve well in liquid are more completely absorbed in the gut than less soluble forms [2,11]. Small studies have found that magnesium in the aspartate, citrate, lactate, and chloride forms is absorbed more completely and is more bioavailable than magnesium oxide and magnesium sulfate [11-15]. One study found that very high doses of zinc from supplements (142 mg/day) can interfere with magnesium absorption and disrupt the magnesium balance in the body .
Here's a good link that can help you find a good supplement: http://www.consumerlab.com/answers/Which+is+the+best+form+of+magnesium+to+take+-+one+that+contains+the+most+magnesium+and+is+best+absorbed%3F/best_form_of_magnesium/