Hi mhailes...
As you've gathered, the coin of choice for this job is the American Half. Mainly because of it's size and visibility (if you get some polish on it, it's VERY silver and eyecatching—perfect for vanishes etc) but also because of it's palmability due to its milled edges.
Other suggestions:
1. Palming coins, thinner edges so you can grip them well (most good online magic stores sell palming coins)
2. Bottle tops (the 'crown' ones from beer/alcopops, NOT screwtops

) are great for practicing your grip. They are super easy to palm, and a good way to train your hand to holding an object without dropping it every minute. Caution: don't practice the muscle pass with bottle tops, it'll end in tears
3. 2p coins are great, but because most of them go dull due to the nature of copper, try using only good shiny ones during performance. Coin work always looks better when you can 'see' the coin
4. For moves like Goshman/Tenkai Pinch etc, it's better to have thinner coins. Therefore, £2 coins make it semi impossible to keep the coin concealed
5. Although this sounds daft, practice coin rolls, vanishes etc. using a hand cream rubbed in all over your hands to make them a bit 'tacky' (don't use too much to make them greasy). I guarantee you will drop your coins less often during coin rolls, and it will aid you in finding your 'sweet spot' for palming
6. Having a good selection coins is a good way to get creative. Chinese coins are particularly good contrast to most others, especially the bright brass/enamel ones with a hole in the middle. Great for transformation effects, and available in American Half Dollar size
7. Don't be afraid to experiment with objects other than coins... casino chips, parking tokens, bottle tops, metal washers etc. are all everyday objects with which you can do impromptu 'coin' work. Keep your options open
8. To more advanced coin workers, choosing coins which 'chink' can be an advantage OR a disadvantage. Some coins chink more than others, which is great in effects such as 'coins across' where the audible value heightens the effect of a coin 'appearing' to move from one hand to another... however, other effects require that multiple coins in one place are less detectible—so chinking is a no-no.
Seige's tip: It's a bit OCD, but try sticking clear laminating film on both sides of a coin. Done carefully and cut out carefully, the coins will look lovely and shiny, but will have a very dull 'chink'.
And also, as a final note, try and keep your coins nice and clean. I find that a piece of kitchen roll soaked with window cleaning fluid (the 'watery' type, not cream type) will get the natural oils and grime off your coins which your hands naturally put on them. Clean coins look nicer, handle nicer, and won't leave 'smells' on your hands.