Few things:
1. Field dressings. Absolutely positively always maraude them from the corpses of friends and foes alike after being shot, before using my own, while possible.
2. LMGs. For ambush or defence, with long sustained automatic fire — MG34, no exceptions. It has decent size drum and very favourable iron sight for defence — you see everything and are able to quickly point and shoot the target. Bren’s iron sights are horrible for covering wider spaces, and in close quarter defence you wouldn’t have much time to react while zoomed. Besides, relatively small magazine size spells many a deaths at close range.
Bren has it’s advantages — being able to shoot from the hip is one of them. Also, it is good for supressive fire — single-shot/short-burst pinning down of enemy sniper/rifleman, while the rest of your squad is advancing/flanking. MG34 could be used the same way too, but it seems to be more effective in short burst at long range rather than single shot.
Didn’t have much chance to practice with Italian gun, still waiting on American, Russian, Japanese MGs.
3. Tripod MGs (Vickers and Lafette MG34). Those are very good mobile supressive/covering fire points. Vickers has iron sights without much zoom, while MG43 has optics, wich helps a lot. However, Vickers can be turned around 360° without much problem, while MG34 has limited lateral range and you have to slowly move the whole lafette using strafe buttons (A/D by default) to turn it around.
Both are very accurate though and even with Vickers I was able to mow down targets that are only as little as 5mm tall on the screen. Better than you can do with standard infantry rifle. Plus, giant-sized magazines.
Negative side of using those is that you present high-profile target for the enemy, compared with prone machine gunner and you can hardly find decent cover. But, unless there are enemy snipers or armored vehicles around, you have nothing to fear from the distance when it comes to small arms fire. At close range though, you have measely sidearm at your defence, so, in squad play, I would also have rifleman or assualt next to the machine gunner to protect him. NCO would work fine for that. Also, NCO or scout could act as the spotter with binoculars for hidden targets (works charmingly in Project Reality as well).
4. Cars with mounted MGs and APC can also be used as mobile supressive fire bases. I recently put emphasis on it in BF2 — you can rack up tons of kills on maps like Jalalabad.