Only been sportsbetting for a year but I'll throw in my opinion.

"Do other punters out there, simply back teams straight out to win or do line betting or doubles, line doubles etc."

Depends on the sport. For NRL/AFL I linebet. Mathematically it shouldn't really make a difference. All my bets are always singles (unless it's a correlated parlay).

"Also, do you bet a proportion of your initial bank each week and increase that as your profit increases"

My unit size is a % of my BR. I resize it once every few days (unless I take a big hit, when I'd resize immediately)

"Finally, do you outlay a proportionate amount each week regardless of what you think the round is like"

No.

"eg 50% of weekly bank on your best bet, 25% on doubles"

Hoping you don't put that much on any one bet...

You shouldn't vary your bet size unless you KNOW your advantage on specific bets is larger. I'd recommend you find a % of your BR that you are comfortable betting each play, and just stick to that.