For us at least, we start Friday night with opposing sides in their trenches. We normally have a few attacks here and there, and lots of smaller raids and patrols. Nothing like low-crawling 75 yards to the German wire, freezing dead every time a flare goes up, and hoping like hell that the 08 Maxim you're working under stays silent. Saturday goes all day with about the same thing. We're on the honor system as far as kills go.. We're not playing cowboys and indians, we try not to get into too many "I got you! No, I got YOU!" arguments. We use smoke bombs (biiiig ones) to simulate poison gas (and we use SBR's or die trying), and people rig up handgrenades, rifle grenades, and mortar projectiles with plastic and cardboard.. Little popping charge of black powder, and some white powder to mark the spot (and kill the fire). The Stokes and Minenwerfer sections are pretty accurate too. We require helmets in the line at all times because of these things.. They stand a pretty good chance of knocking you silly WITH the tin lid on, but it'd be worse without. Oh yea, and the barbed wire is real too.. Just farm grade, nothing like the THICK stuff they used in the war, but falling into a 4 foot high, 5 deep belt of it sucks just the same.
www.great-war-assoc.org has a video on it that has a little "combat" to give a taste.