Vertical position
Vertical MIG welding, both up and down, can be difficult. This makes pre-weld setup very important for making high quality welds. Since you are fighting gravity, consider reducing the voltage and amperage by 10 to 15 percent from the settings for the same weld in the flat position.
The vertical down technique helps when welding thin metals because the arc penetrates less due to the faster travel speed. Because vertical down welding helps avoid excessive melt-through, welders sometimes place very thin materials in the vertical position even if they can weld them in the flat position. When welding vertical down, begin at the top of a joint and weld down. For thin metal where burn-through is a concern, direct the wire away from the weld puddle. Keep the electrode wire on the leading edge of the weld puddle. A very slight weave may help flatten the weld crown.
With the vertical up technique, begin at the bottom of a joint and weld up. This can provide better penetration on thicker materials (typically 1/4 inch or more). The travel angle of the gun is a 5 to 15 degree drop from the perpendicular position. A slight weaving motion can help control the size, shape and cooling effects of the weld puddle.
Overhead position
Drag, push or perpendicular gun techniques can be used for MIG welding overhead. But because of gravity, travel speeds must be fast enough so that the weld metal does not fall out of the joint. Also, for this same reason, weave beads should not be too wide. Lowering the voltage and amperage can help keep the weld puddle small and more controllable, which is why you might want to consider using a smaller diameter wire.