Hey @deanliu, I usually have multiple targets on a trade (whether $/BTC or BTC/altcoin), so I will scale out of of the position along the way, I never go 100% out of a wining trade because it might always go higher and you end up being low-tipped, actually I cover this strategy in one of my old articles:
https://steemit.com/cryptocurrency/@tradealert/strategies-to-trade-with-the-smart-money
As far as exchanges I use BITTREX and Bitfinex for altcoins and Kraken to buy/sell BTC for fiat money. I would stay away fro POLONIEX right now though, too many red-flags :)