I started off going about this in the wrong way, and that was by auto applying skills...
The problem with that, is that some players get apparently very good... But at the most random attributes... I have a 10 who is a good scrummager?!?! 
Really there are a few different ways to look at this, I then started to build up speed and penetration (giggedy), so I ended up with great attackers, but poor defenders, so then I upped tackling and defense speed, but then my.star players got injured ALL the time! So then I spammed toughness, and now I tend to stick with personalised improvements.
But to start again I'd say:
front row is scrums and toughness 
Second row is line-out and toughness
Back rows is tackle and penetration and toughness 
9 passing and toughness
10 kicking and toughness 
Centres penetration and toughness and tackle
Wings speed and toughness 
Fullback tackle and toughness 
Ofcourse don't neglect other attributes, but those seem to be the most logical in my mind... I always did toughness when I realised about the injuries because if they're injured then they make 0 training points, even if they play badly, they still get something...
Always play tournaments too, as you get more money and training points...