Good ol' Radio Shack 18 guage solid core hookup wire run loosely in parallel, with soldered banana plugs at the ends. I read about using this wire for speakers years ago in Stereophile's recommended component list. When I tried it, I heard a night and day improvement over my generic 14 guage stranded "speaker wire."
18 gauge is right at the threshold of not being too thick to allow high frequency skin effect to cause much phase delay between bass and trebble, and solid core solves the problem of polarized oxidation layers between the stranded wires. Running them loosely without twisting them keeps capacitive coupling very low.
But what about the damping factor robbing in-line impedance? Depending upon the length of your run, this solution would work best running lower powered speakers.
Heh; that's where I have improved things a bit. I specified KIT or DIY speakers. The crossovers for my speakers are now housed in a separate EXTERNAL housing that sits next to the amp. Wires run a short distance from the amp into the crossovers, where the signals are separated and run via separate wire pairs to the speakers in a pseudo bi-wire arrangement. Depending upon your crossover point and speaker impedances, this will divide the current by varying degrees between the wire pairs, lowering inline impedance while keeping the traveled distances the same. All this without needing air cores, kevlar sheilds, or hyperlitz configurations.
I get EXTREMELY good results by using this. My speakers image like the dickens, and bass is taut. However, my speakers are all acoustic suspension with high damping coefficients, and I'm only running 100W into high impedance loads so I this arrangement may not be suitable for other systems. I also have no pretentions that my sound wouldn't be better with expensive wires. What I do know is that I'm at a point on the diminishing returns curve that upgrading speaker wire in my current system is unthinkable.
For the cost, it's worth trying. You can take the crossover temporarily out of your DIY speaker and move it behind your amp; you should have separate input jacks for bi-wire options anyway, so no cabinet cutting should be necessary. If you're currently running thick stranded "speaker wire," you may just be surprised at the improvement. If you're already running expensive speaker wire, you're probably well beyond such cheap tricks so move on to another thread and stop being so nosey.
My disclaimer: If you try it and think it sucks, post your experience here but know in advance that I refuse to be liable to refund you the $2.98 you spent.
