Early-game Principles I am a novice player who is trying to brush up on the core principles for successful chess. 

Instead of focusing on specific openings in the beginnings of a game, I am trying to play to certain principles, namely:


Control of the centre by a combination of pawn and minor pieces.
Early development of minor pieces
Not losing momentum, or hindering development, by having to move the same piece twice.


Specfic Problem: 
I am currently having trouble with tripping over myself by moving pieces in the way of each other so that I end up not adequately defending pieces, or defending pieces come under attack.
I often lose a pawn in the center which I was trying to defend, i.e. I failed in taking a square I wished to hold (d4, e4). This tends to happen by a pawn being taken and then the attacker pawn is defended by another pawn, so that my developed minor pieces can't retake without losing material in the exchange. So I think I might be pushing forward too early with my pawns. (I usually move e4, then some variation of knights, then bishop. I also don't know where to place my bishops (the only places I know for tactical/strategic advantage are pinning an opponent's minor piece against queen or king, or to aim them at the opponents weakest square (f2, f7).

General questions:
Are there any other early-game principles? What order do you generally develop? How many turns does it take you to make a push for the centre? What squares do you move your minor pieces to for strategic or tactical opportunity?
You can probably benefit greatly by improving your tactical calculation. Doing some exercises on chesstempo.com will help you get used to calculating out those exchanges. What you are describing seems like a tactical issue and generally can't be addressed using strategic ideas. 

That said, a good way to avoid those difficulties may be to specialize in some less complicated openings until you get comfortable with the tactics that appear in the opening. Maybe the London System (1. d4 ... 2. Nf3 ... 3. Bf4) for white and the Center Counter Defense for black (1. e4 d5). Both have some traps, but there are less to deal with. 

Personally, those are the two openings I liked to use to get "used" to chess without having to really deal with the openings much. Especially if you are a more strategic, positional player, these two openings should feel pretty natural.