**EDIT/UPDATE - It wouldn't let me add more pictures in edit mode so here ya go.
BUNKER IRRIGATION/RETAINING WALLS
Anytime you dig out a bunker, which can sometimes take hours or hard work, you don't want your hard work to get ruined with the first storm. So in order to keep your trenches in top form, I would suggest using a bit of forethought to keep them from filling back in or collecting large amounts of water.
To start with the best way to keep them from filling back in is to construct a retaining wall of sorts. What you can do is get some 2x6's at the hardware store and lay them down on the floor of the bunker/trench. For the actual wall you'll need some braces like rebar or metal fence posts of some sort, then make sure when you're digging to make the walls slanting backwards so they fan out a bit. Then lay your 2x6's accross using the braces to hold them in place.
Now that should keep the dirt from falling back in but what happens when it rains horribly bad and you've got a mini lake? Well to make sure you get some drainage you can dig out your bunkers so that they angle back and taper out so that the water has a channel to flow out of. To keep all the dirt from washing out of the bunker I would put some loose gravel or stones under your 2x6's. If you wanted to get really fancy you could even get yourself some drain pipe systems.
FIELD SIZE AND LAYOUT
When it comes to field size it really depends on what you're working with. I try to keep in mind that it's not a good idea to be able to long ball eachother while you're sitting in both the main bases. That means the main bunkers have to be at least 250-300 feet apart. But in most cases there will be moderate brush and trees that will keep you from doing this in the first place. This is just a rule of thumb I like to keep in mind when thinking about large and small fields. When you're playing woodsball games like capture the flag it's always nice to have some room to maneuver before you engage the enemy.
Beyond that it's all going to be personal preferance as to how big you make your field and where you put the bunkers.
How big should a field be? Well in my opinion for anything smaller than 5 on 5's 2-3 acres will do you well, but 6 on 6 and up would require a little bit more room(just my opinion). I would say a good field size to satisfy both large and small games would be around 3 acres. When you start getting in to 15 on 15 and bigger you're going to need a bit more room, like 5-7 acres. If you've got a lot of brush and trees standing between both bases you can make the field a bit smaller, but if it's all flat land with little in between you're going to want to give it a bit more room.
When I think about designing a field I usually like to think of having a main path or road that goes through the middle of the field and connects the two main bases with several smaller trails that sever the other sections of the field and connect some of the smaller bunkers. This gives you clear routes to each base and also gives you some cool options for scenario games.
As you can imagine this is just a rough design for a field. Depending on what the terrain of your field looks like this design might have to be modified quite a bit. What I was going for though is just a general rule of making both sides equal, keeping a balance of defensive and offensive bunkers on each side. Always try and think of how a bunker could be used both ways when building them because even the best defensive bunker COULD be used against you if you're not in it(yes they can hide on the OTHER side of it just as well as you can hide on the inside of it). Don't make one sides defense impenetrable and one side too easy to take over. Also keep the elevation of the field in mind like if you're putting all the other teams bunkers on the low ground and all of another teams bunkers on the high ground.
If you're going to construct a field with a 3 base design(2 main bases and a middle base for playing middle flag) Make sure you make the middle base of equal distance from both main bases. I usually like to stand in the middle of the field between both main bases and if I can just barely not reach the other base with a normal marker(not flatline) it's just the right distance away, but if I can pelt it then it's too close. Usually that's going to be about 150-175 feet, making the whole field about 350 feet apart or about 1.5 acres.
CLEARING BRUSH - Brush can be good, but too much brush can kill your field. You want to have just enough to give you lots of cover but not so much that you cant see anything until you're 3 feet away from it. Don't be afraid to clear some brush! If you mess up and clear too much, don't worry it'll probably grow back by next year anyway.
SANDBAGS AND TRENCHES - I thought I would add another diagram real quick tonight to cover sandbags and trenches. It's pretty self explanatory.
With trenches you want to keep in mind that they'll probably need to be at least 4 feet deep and 3 feet wide. Trenches make things a lot more interesting but due to the amount of digging probably wouldn't be too practical to use for ALL of your bunkers. I would suggest using a mix of all the different kinds. The trenches would work well as forward or main bunkers because of the low profile and low exposure to offensive forces.
The plywood boards just add a sort of realism that makes it a bit cooler. It also helps to keep things safer so you can run accross the trenches without having to worry about falling in. I would think about having some ground level planks running accross the top of the trenches in a few areas where you can run accross. If you're going to do this then the planks would have to be flush with the ground and the trench would have to be gradually dug out deeper in the spot where there's a "low roof" so you're not running along and WHACK! catch a board in the forehead.
I'll have to add more later when I'm not so tired and more coherant.
Let me know if you guys see anything that looks off or you think should be updated.
**UPDATE - LOG/BRANCH BUNKER DIAGRAM
Well I realised I had forgotten all about the first bunker I ever built, the good ol' log bunker. All it takes is a hatchet(or a kabar as I used to make mine) and some elbow grease.
For this bunker, the design is simple. For posts/stakes and then a series of logs stacked up in between them to form a wall. Where do the branches and brush come in? Well that's what you use to fill in any significant gaps.
To start just find yourself some over grown christmas trees and hack em down. The stakes/poles ares going to need to be about 6 feet tall, so make sure to cut them at about 6 and a half feet to leave room at the end for sharpening. Sharpening the end of the stakes makes it easier to pound it into the ground. Another note, a good size pole is gonna be about 3-4 inches in diameter.
When you pound the stakes/poles into the ground make sure they down "Y" out but sort of slant towards eachother(even crossing eachother towards the top works well to keep the logs in place).
Then all you have to do is find some good logs(maybe about 5-6 inches in diameter - too big and it'll make it a bit trickier) and stack em up in between the posts.
Here's a couple of pictures of the first bunker I built to give you a better idea:
I apologize for the poor picture quality.