Natural Calm at the recommended dosage works great for me.

i take 400-600 daily and have found this to really help my stress, anxiety and sleep. i take 200-300 in the AM and the same thing before bed.

i would start at 100mg 2x a day and build from there, if you take too much at once it'll make you poo.

I've found the 250mg dose 1x/day to work the best for me. Taking the 500mg messes my entire system up--can't sleep at night, feel lousy, etc. If you're eating enough fresh fruits and veg, you should be getting plenty of Mg. But I understand that sometimes you need a bit extra to cope with the stressors of modern life. Keep in mind that the average adult needs 300-400mg/day (RDA). That's why large-dose supplements are not the greatest idea, your body needs time to process/convert all of it. Best to spread it out throughout the day.

http://ods.od.nih.gov/factsheets/magnesium

From Wiki: Excess magnesium in the blood is freely filtered at the kidneys, and for this reason it is difficult to overdose on magnesium from dietary sources alone.[18] With supplements, overdose is possible, however, particularly in people with poor renal function; occasionally, with use of high cathartic doses of magnesium salts, severe hypermagnesemia has been reported to occur even without renal dysfunction.

So basically, your kidneys have to be pretty messed up to lack the ability to filter magnesium.

If you are going to supplement magnesium have your doctor check for kidney function.

These are the instructions I use. However this is for maximum dosing and you may not want to go this far. Some folks suggest taking on an empty stomach which I do.

Choose the amino acid chelated form of magnesium glycinate identified on the label as an Albion patented product. Check label to be sure it is not a combo product including calcium or other minerals. This amino acid glycinate has an 87% absorption rate. Typically, these capsules will be 100 or 150 mg.

I use: http://www.iherb.com/Doctor-s-Best-High-Absorption-Magnesium-100-mg-120-Tablets/15?at=0

Ramp up or titrating up the dose from a small amount – one capsule a day…increasing every 4 – 5 days --- very slowly to allow for tissue saturation without causing bowel intolerance. The chelated amino acid form is specifically formulated not to cause bowel intolerance until dosing levels are quite high. The goal dose is 600 – 800 mg daily in divided doses. This form of magnesium does not depend on stomach acid for metabolizing so it can be taken with or without meals. Start with a low dose and titrate up very slowly so the tissues acclimate to the magnesium and eventually become saturated… this will be signaled by two soft bowel movements a day. Obviously, back down to a lower dose if diarrhea occurs. Diarrhea is counterproductive because electrolytes are lost.

Example - start one 100 mg capsule at bedtime…for 4 – 5 days….then:

Increase to another capsule at breakfast - continue 4 – 5 days, Add another 100 dose at lunch; continue 4 – 5 days; Add another 100 at dinner making a total of 400 divided over the day and evening. Then increase again starting with 200 at the nightly dose - wait 4 – 5 days etc. Always be very aware of loose stools and cut back immediately. You may have to coast a bit on a lower dose before trying to increase again… but eventually, you’ll find a dose that works without causing bowel intolerance. Everyone is in different stages of magnesium depletion and usage, so it’s simply a matter of trying and keeping track.

The very worst thing to do is take a large amount of magnesium all at once. That doesn’t accomplish the goal. It takes about 120 days to recycle old blood cells with new so results many not happen quickly.

The below book goes into much more depth:

The Magnesium Factor Magnesium researchers, Mildred S. Seelig MD. MPH. Andrea Rosanoff, PhD.