M.L.
I'm a stylist. Sounds like your natural wave pattern in the back is grouping your very loose curls together as the day progresses, could be caused by sweat, oil production, or like Janelle suggested too heavy of a conditioner or not fully getting it rinsed out.
I would use a very lightweight volumizing shampoo/conditioner and mouse since your hair is so fine. Body Double by Sebastien works wonders, it's expensive but very concentrated so you only need to use a little bit at a time.
http://www.haircareusa.com/sebastian_body_double_thick_ha...
I've never used, but heard good things about kms Flatout for smoothing fine and thin hair:
http://www.haircareusa.com/kms_flat_out.htm
You can velcro roller your hair to get some volume and I always bring a comb or soft bristled brush when I go out to touch up my hair (mine tends to twist and get tangly too!). An amazing brush that really is good for smoothing and high shine is called a denman. But most natural brist;e brushes will have a similar effect.