Using both together gives you the most accurate and up-to-date picture of what your card is actually worth right now
Once you know more or less what you can expect for your card, visit your local card shop and ask them what theyre willing to pay for it
Meaning cards that count creatures (e.g., Shamanic Revelation, Pennon Blade, Gaea's Cradle) count her
You have to reduce your colourless land count in order to house these effectively but otherwise you shouldn't get burned by having one or two of these in your deck
Ideally, you have something for cascade, graveyards, Burn, big mana, and blue mirrors
Dark Ritual MTG Gatherer really knows how to throw a wrench in my memory of Magic: the Gathering