MelC,
When you start in the corner, do you work up to the top or go side to side?
Once you get the wrinkles out, use two fondant smoothers, one in each hand, and bring them towards each other at the corners and edges to help prevent the tendancy to round off corners when using just your hands. I hope I'm describing that in a way that's understandable. LOL, just because I know what I'm talking about doesn't mean anyone else does! 
When I learned that tip, it was like a lightbulb moment for me. I use that method on any square cake I do, even if it's buttercream and I use the two smoothers over a Viva paper towel.
Deanna
I start at the top of the corners (after I smooth out the top of the cake)... work my way down them about 2/3s of the way down... work the middle (flat) part of the sides a little... finish the corners... then work the middle the rest of the way down.
If you start with the flats, you'll just end up with a whole lot of fondant on the corners that you'll never get worked in (I always ended up having to cut some off, then mess around with the seam!)
If you were to start at the bottom, you're just about guaranteed to have air bubbles under the fondant!
And I'm sure you've all heard it before, but I always use some olive oil or crisco on my board & hands & rolling pin... never corn starch or icing sugar! I had too much trouble with the fondant cracking and sticking with the dry materials!
great job on that heart cake. It looks so smooth and pretty!
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