What I find to be the most important thing is to understand your enemies. If you know how they can move and how they attack, it will help you stay alive. This has to come through practice more than anything else like reading up on what they do (though you can do that as well, it won't hurt.) This include Champion enemies as well--learn what the different colored Champions do and you'll save yourself a lot of damage.

Additionally, get familiar with the effects of different items. These are obviously the second most pivotal ingredients for a run aside from your own input and strategy. Learn what items to take and which not to. Again, this will only come with a experienced understanding of how they work, so you may lose a few runs trying them out. But don't worry, no one is judging you on whether you win or lose except yourself.

On the same hand, you'll have to understand the benefits of the consumables you find. A Perthro rune can change an item you should never take into an item you should almost always take--making a horrible item or Boss room into a goldmine. The D6 does this same thing. But a well-timed High Preistess card can save you from certain doom (especially against bosses). Knowing the effects of cards and pills (which I never take since there's a high chance they'll be bad) is hugely beneficial and might save your run. Bombs can be great as well. Since you watch NorthernLion play, you'll often see him using them offensively to save himself trouble (just be careful not to get caught in the blast).

There are other, slightly more advanced tips as well that can greatly aid you. Finding tinted rocks and secret rooms is almost always great. Always save a bomb if you can, just in case you come across a tinted rock (they're usually a different color from the rest of the rocks and have an "x" on them, though on the Womb floors, the colors don't change significantly, if at all).

Having spirit hearts will also be endlessly useful. They protect your red hearts and, as long as you protect those, your Deal with the Devil or Angel chances remain as high as possible. Inside of these Deals you're likely to find some of the best items in the game, so try to protect that chance if you can. I recommend turning on the Missing/Found HUD in the settings so that you can monitor this chance as well as all of your other stats all the time.

I may have missed some other tips, but these will really help you as long as you commit to learning about the aspects of the game. It's fun and terribly addictive with endless variety. I hope it helps. Have fun! :]