question: I haven't done any margin trading or shorting on bitfinex yet, but I am considering shorting LTC/BTC at some point in the future.
Does it matter in what form of money my margin is? (i.e. do I need USD, BTC, LTC ?)
Any peculiar things or common mistakes I should look out for when trading on margin? (for example how do you partially close a position etc.)
The denomination of your margin doesn't matter, the current value is what matters. Since the current value is calculated dynamically, determined by the current rate of BTC/USD and LTC/USD (correct?), changes in the overall value of your collateral (=margin balance) are something to keep an eye upon, since any change here is leveraged: If the value of your collateral increases, your tradeable balance increases multiplied by the leverage factor, giving you more "fire power". But the contrary is also true (and a dangerous pitfall). A decrease in the value of your margin balance can drive you into forced liquidation much quicker than you'd expect.
Thus it's a good idea to keep some "headroom" on the tradeable balance. This allows you to transfer small unused portions of your margin balance temporarily to the exchange account, and change them into another kind of underlying (eg. sell BTC for USD when you expect falling BTC rates, and transfer the USD then back to the trading account.)
You reduce the size of your trading position with trades in the opposite direction. Only when you close your position completely is when your gains (or losses) are credited to or deduced from your underlying trading account. A common pitfall here is not to take all your open limit orders into account. For example, if you short, you typically set yourself limit orders at some point below to take your profits. Now, when the market drops suddenly, all your limit orders might be executed; if they sum up to more than your current position size (e.g. because you closed off some part manually), the direction of your position will be turned around, i.e. in this example situation you'll be left with a long position over the difference amount, and this might happen in a still falling market while you're sleeping.
LTC/BTC is indeed an interesting segment, but beware, the LTC market on Bitfinex (and still more the LTC/BTC segment) is lacking depth. The situation has improved significantly, but this is still largely an "illiquid" market. It can easily be controlled by some large positions, and it can happen that it doesn't follow the general trend, just because some large player doesn't like that trend. But most importantly, the depth might not be enough to get out of a losing position in case of a sudden trend change.
Of course, the general advice for any trading applies: trade the
trend, not the
actual levels. Never try to
anticipate developments (this is gambling, not trading). Know your exit point beforehand and never argue about it. Choose the size of your position based on the maximum
loss you're willing to take
right away, not by considering prospective gains. As we all know, this kind advice is easy to state but hard to follow.