Not all black sand is magnetic, for instance somewhere like Eldorado in Vic probably only 5% (or even less) would be magnetic so using a magnet in that instance won't help much at all. Other places may have a much higher content of magnetic black sand so it really depends on where you are prospecting as to how much success you are likely to have with a magnet. Also be aware if you have black sand cons that has a really high amount of magnetic content when you use a magnet to pick it up you still need to be careful that you aren't picking up gold with it. Although gold itself is not magnetic it may get caught in the clump of magnetic material as it attracts and travels to the magnet.
A bit like AGMAN I too now use a Garrett 15" Super Sluice pan however, instead of panning a tablespoon or so at a time, I put a day or two worth of sluicing cons in the pan at a time, work it down before pulling a little bit of the top layer back to the other end of the pan (wash back method) and sucking the black sand up with a snuffer bottle. I've found this to be a hell of a lot faster than going a tablespoon at a time, but do whatever works best for you.
You can also double check the black sands you suck up into the snuffer bottle by dumping it into a second pan.
I'm currently working on a mother load of 5 days worth of sluicing cons in the pan at once, let me say I've learnt my lesson that that much in a pan at once is way to much LOL
It was very hard going at the start because the pan was so full I basically had no where to wash back to. Its been slow going and I've checked every bit of black sand I've sucked out with the snuffer and so far zero gold found which means its all still in the pan. The upside is there should be a pretty nice ring of gold in the bottom when I finally reach it LOL! Also remembering that this gold all came from Eldorado so there is going to be heaps of tiny tiny gold in there as well, which shows that this method of recovery works well with even the finest of fine gold.
Cheers
Adam
p.s. you might be able to save yourself some money and pull apart an old computer hard drive, as you'll find two really strong rare earth magnets inside.