Here are some tools and pictures who'll give you a view on how to keep your map clean and focussed. Keeping yourself to this mapping code will help make maps better and faster, find scenes more easily, prevent LEAKS, reduce compiling times and amounts of w_poly.

Tools

1. Carving
My message on the Carving Tool is simple : DO NOT CARVE!. The tool is easy-to-use and can make shapes really fast and thus is popular by newbies. Although the carving tool often messes up the structure of your maps and will create additional poly's everywhere. Your maps will be futher away from home with this tool. If you want a hole in a wall, then just put 4 blocks together rather than making a hole by carving a block in between.
2. Hollow
Same message as the carving tool : It's easy to use, creates fast what you want but do not use it. It's better to create your own box with 4 walls a floor and a ceiling because you have full control over the size of every single wall. Hollow will often make walls beyond the lines of the grid and that will result in a mess, not to mention what happens as soon you start resizing this grouped box.
3. Clipping Tool
Clipping Tool is your friend. If you keep attention to what you are doing and keep an eye on the grid, this tool is great. You can easily shape a box into any form you want. Next to this, it's user friendly because it will only affect the object you have selected. You'll have to practice a bit with this tool, but when you got the hang of it you'll be using it very often.
4. Vertex Tool
Even bigger friend than Clipping Tool. The Vertex Tool is your BEST friend. Again this is for advanced mappers, you'll have to know what you are doing. You can select a box and move all it's vertexes in the shape you want them to be. Just be carefull that you do not create any illegal shapes that HL can't handle. Use the "Check for Problems" option in your Hammer Editor to see if your shape is right.
5. Rotate
Good tool, but be carefull with it. Don't go rotating walls everywhere because that will surely cause leaks. Keep to 90,180,270 degrees rotation to make sure you are not messing your map up. If you want a 45 degrees rotated corridor I would advice you to move the vertexes in the right place with the Vertex Tool.

Grid and Textures

1.Grid

2.Texture Faces

3.Texture Scaling

4.Floors and Ceilings