First off, this isn't a guide to teach you how to build like me, Phong, Izzo, Moko, squieu, or others. I will be focusing on general building techniques and aesthetics. That said, I hope this will help you improve your Lego mecha building skill.
Visually appealing mecha draws from fantasy. There is just something fantasy-like about a giant, anthropomorphic death-dealing machine. A great mech could be flexible like a human, strong like a gorilla, menacing like a spider, fast like a horse, and so on. Mechs are often regarded as awe-inspiring things or even demigods. In many ways, mechs reflect mythical creatures.
Lego mecha construction starts with good joint building. Ideally the joints should be able to rotate in two dimensions. Make sure your mech is flexible enough! There are many ways to go about making joints.
The best joint in my opinion is the Technic T-piece, which is actually a pneumatic part. You can buy these from Lego Pick-A-Brick or BrickLink or wherever for relatively cheap. Here is a good example of how T-pieces are used as joints (the grey pieces): http://www.brickshelf.com/cgi-bin/gallery.cgi?i=3166582
Using T-pieces in conjunction with lamp holders, headlight bricks, 1x1 plate/tile clips is the preferred way to go. The T-piece and 1x1 tile clips have a lot of friction, making it one of the most solid and compact joints.
If you're going for a larger scale mech, use click joints. If the mech is too heavy to support its own weight with regular joints, click joints are the way to go. The most flexible click joints come from the Exo-Force and Knights Kingdom sets. There is also a large Technic click joint for those enormous mechs.
There are infinite possibilities in detailing your mech. There is no "right" way to add detail. However, I will go over some things which are hallmarks of unskilled or lazy building.
Leaving too many studs visible is a major problem among newbies. It's fine to not cover up studs, but typically this is not visually appealing on a mech. Cheese slopes, round bricks, and tiles are great pieces to cover up studs with. Armor is typically flat and should be studless, while grass is textured and it is better to leave grass studded. Studs vs. studless depends on what you're trying to model, but due to the generally flat look of armor plating, studless is the way to go.
Shape and proportion are often difficult to master for less skilled builders. Try to measure out how long you think each body part should be, and then go about building it.
One thing I would recommend is to try to make a mech with the smoothest shape possible, then add color accents, greebles, and odd shapes from there. Often times, less skilled builders will start by slapping on a lot of pieces which don't complement each other very well and call it "detail." No. Don't do that. Be deliberate in how and where you want to add visual appeal.
A good build should reflect your personal building style (*ahem* I'm talking to you Phong). This may take a few months to a few years to master. Not many people start building Lego and put out an amazing build on their first try. Building does take a lot of practice. Permalink