I'm breaking from the mold with these final positions because frankly, drafting for these isn't so much about strategy as it's about just filling the position. At least, that's the thought you should be having regarding kickers and defenses.
The ongoing theory in drafting kickers is this, don't do it till the end of your draft, and I must say that's a pretty sound rule to live by. If you look at the projected fantasy points for kickers (and again, I'm using cbssports.com for this) the difference between the top projected kicker and the 20th is only a measly 23 points. In fact, you have to go down to the 28th kicker before you get past a 30 point difference. When you consider that you could be using earlier picks for added depth to your roster or for handcuffing your studs, then it really is pointless to draft a kicker before your last picks.
Like the kicker, defenses (including special teams) shouldn't be a very early or even a mid draft choice. While they put up considerable points in the grand scheme of things, the difference between the top one and the 12th is a miniscule 65 points. Take that to the 20th defense and it's 78 point separating them and the top. Unless you can nab the Steelers, Ravens or maybe even the Packers D, then you can really wait till the last few rounds to pick your defense. But keep this in mind, even if those suggested D's are available, don't snatch them until you've already secured your starters (other than kickers) and some solid back-ups. In the long run, those will be worth more.
The flex, as defined in fantasy football terms, is a spot in your starting lineup that you can use more than one type of position player; typically RB and WR. Although some leagues' flex spot allows TEs or even QBs.
If your league uses the flex and it's open to all positions, then I recommend placing a top back-up in there each week based on matchups. If the league is like the one I'm currently in that has you draft players to your flex where they can only be used for that position, then I recommend placing either a QB or a running back in there. Both of those positions touch the ball more than a tight end or wideout and when you're talking about potential scoring, you want the player around the ball the most to be in that spot.
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