I use many sources including coinmarketcap (look closely at market cap data), coingecko (concisely summarizes development activity), coin telegraph (for industry wide trends), forbes, btcmanager, themerkle, twitter accounts of respected industry professionals, e.g. Charles Lee, Roger Ver and my most important source is this bitcointalk forum. There is a wealth of information here especially if you read messages posted by Hero and Legendary members.

I also look closely at charts to time entries and exits.