I was struggling witch my barre chords, but then  my teacher showed me two great exercises. I've been doing them for a week or two and - it's a miracle - barre chords started sounding!

#1. Pure barre practicing

Just hold all the strings on fret 7 with your first finger and nothing else and check if all the strings sound clear. You can help with your second finger. When all notes are clear, move to fret 6. And so on until you reach fret 1.

#2. Tough barre exercise
Hold your barre on fret 7 and with the other three fingers do this pattern picking one note at a time:

    -------------------------------------------------------------------8-7-9-7-10-7---
    ------------------------------------------------------8-7-9-7-10-7----------------
    -----------------------------------------8-7-9-7-10-7-----------------------------
    ---------------------------8-7-9-7-10-7-------------------------------------------
    --------------8-7-9-7-10-7--------------------------------------------------------
    -8-7-9-7-10-7---------------------------------------------------------------------

Keep holding the barre on fret 7 for the whole exercise! By the time you get to string 1, your hand will hurt like hell, but that is the purpose - to get your hand strong. Rest for a while, and do the same thing on fret 6. and so on until fret 1.

Do this every day for 2 weeks - and barre chords will go like nothing.