hi, as an entry point into plato, i'd suggest this book on crito with lots and lots of help, including revisions of the perfect and other points covered in the 2nd-half of textbooks (when students may hit brain overload):http://www.amazon.com/Greek-Prose-Cours ... 185399538X
for the pre-socratics, you're usually dealing with fragments/quotes without surrounding context and so it's a different approach altogether, you need to do this on a philosopher-by-philosopher basis and so you'll need to gather as much context as you can from a whole range of other sources. e.g. i have a good commentary on parmenides by conche which i picked up here in paris in a philosophy bookstore (vrin), but for surrounding context i'd suggest something more general on the history of philosophy, i'm guessing you already know the good ones, i personally prefer bertrand russell's history of western philosophy but one of my bosses at work is heavily against it, take whichever you prefer.
one thing to really get comfortable with to read the pre-socratics is dialectical differences, you need to be comfortable with reading non-attic to read the pre-socratics, the pre-socratics were in western turkey, southern italy etc. and so in e.g. heraclitus' ἀνθρώποις γίνεσθαι ὁκόσα θέλουσιν οὐκ ἄμεινον, a line i really like, an ionic word like ὁκόσα would be ὁπόσα in attic, things like that. cheers, chad