Dogface makes good points here i think more sixman games, and well really just football games are lost simply due to poor tackling open field especially in sixman because alot of times those LB and DB are left on an island and are the last means to make a tackle,
i think working on flowing to the ball is also a big thing to work on. Getting off of blocks and team pursuit and gang tackling is a big deal too.
as far as reads are concerned Jc made a good point for the corners in the 2-3 but it really depends on what type of D you are running
some basic points some im repeating from earlier
form tackling
open field tackle
pursuit to ball/flowing to the ball
backside defenders to contain cutbacks
reading and identiying ball movement
awarness if the LB is so busy watching the ball he is going to miss the linemen sneaking out on a pass pattern
getting off of blocks (cant tackle if the lineman whips you and your downfield 10 yards)
communication