One of the easiest ways to make methamphetamine is from amphetamine. Of course, this assumes you have amphetamine in the first place, but let's just pretend you have some and you want to spice it up a bit.
The difference between amphetamine and methamphetamine is the addition of a single methyl group (CH3) to the amino group sticking off the middle carbon atom in the chain. Fortunately, substituting amines is really simple. Vaporize your amine (your amphetamine) with a bunch of vaporized chloromethane (CH3Cl, a solvent) and some gaseous pyridine... voila, the amino group takes the methyl from the chloromethane and lets a hydrogen go. The hydrogen joins the liberated chlorine, and the resulting HCl is soaked up by the pyridine. The pyridine is optional. Adding it drives the reaction a bit by pulling the excess HCl out of the equation, but it's not neessary.
Assuming you don't have amphetamine lying around, an easy synthesis with a very high yield is to reduce the condensation product of phenylacetone and methylamine. The benefit of this method is that different amines can be used to produce novel N-alkyl amphetamines (ethamphetamine, tert-butylamphetamine, etc)
Making it from ephedrine or pseudoephedrine is possible. The only difference between methamphetamine and (pseudo)ephedrine is that damn alpha-hydroxy group. Reacting your ephedrine with thionyl chloride replaes the OH with Cl to produce N-methyl-alpha-chloroamphetamine as an intermediate. Hydrogenating this product is easy: use lithium aluminum hydride, sodium borohydride, or even hydrogen gas with nickel or platinum metal as a catalyst. The product of this step is N-methylamphetamine and HCl. Evaporate off the water and you have methamphetamine hydrochloride.
A surprisingly simple synthesis is possible from the amino acid phenylalanine, which is available at health food stores for about $14 for 100 tablets. Phenylalanine is 2-amino-3-phenylpropanoic acid, which is more or less amphetamine with a COOH where the CH3 should be at the end of the chain. Thionyl chloride will replace the OH with a Cl, which falls off and is replaced by H when you give it lithium aluminum hydride, sodium borohydride, or hydrogen gas and nickel/platinum. If you use hydrogen and metal for that step, you'll have to reduce the carbonyl group with one of the hydrides, so best save time + effort and use them and do both reductions at once. When that carbonyl is reduced, you now have amphetamine. Go back up to that first one I mentioned for upgrading amphetamine into methamphetamine.
Note that azll of these (and probably anything anyone ever comes up with) will give you a mix of d- and l- isomers. The d- is cool, the l- is shit, remember. If you have time, energy, and equipment, you can separate the two and reprocess the l- into d- by oxidizing it and re-aminating it as described in the "critique" of the Phrack synthesis.