Good power is required. Some use extension cords to load down their spotwelder and make it a bit more controllable. I plug DIRECTLY into the outlet. Takes me two seconds to do a perfect weld. Your mileage will vary, depending on your circuit, as well as how far the outlet is from the panel (more wire, more load). Practice on scrap. I used old tapco recievers to get the hang of it. By the third one I had the process down.
Others mentioned SERIOUSLY cleaning the parts to be welded, as well as keeping the tong tips clean for each weld. Listen to them, they are absolutely on the money with these tips/tricks. Clean inside the reciever, outside, inside the rail, and outside. I use surgical gloves during cleaning/prep, and a new set afterwards to prevent the oils from my skin contaminating the weld areas. I use 3M red scotch brite pads to scuff up the spots I'm welding. Plus it works VERY well at getting into the channel for the lower rails with a small screwdriver. Get them at the auto parts store near the paints.
The BIG thing nobody really talks about is proper adjustment of the spotwelder tongs themselves. Too tight = too much resistance = bad weld. Too loose = not enough contact = bad weld. The tongs should be just a little snug, NOT super tight. Let the c-clamps do the gripping, NOT the spotwelder. Read up on the Miller web sight, especially the spotwelding pdf. A TON of important info for using spotwelders there.
Check sections 4-8 and the appendix for setup of your spotwelder.
http://www.millerwelds.com/pdf/Resistance.pdf