I wet blend metallics a lot.
The best method I've found is to be sure of your progression; so lay down blocks of the appropriate colours first. You'll generally want your shade to be less reflective than your highlight, or light glare will ruin the effect.
Let's take the gold on my titan as a pretty good example:
This is done by first putting on a nice smooth basecoat in the approximate midtone (here VMC bronze) and applying a brown wash just to knock it back a bit.
I used the following colours:
VMC Black
VMC Bronze
VMA Aluminium
GW Devlan Mud (Agrax Earthshade works just fine too).
You'll want a decent size wet palette, and a hair dryer.
On the palette put a smallish blob of black, a big blob of bronze and a small blob of the Aluminium.
I then mixed black with the bronze (making a blob between the black and bronze on the palette) and applied that in a block to around a third of the progression.
Mixing a little of the aluminium with bronze for another mix blob between the aluminim and the bronze. Aiming here for a lighter gold (but not too much brighter). That goes in a block on the other side of the progression.
Dry those with the hair dryer. These are to roughly put into place where your 'targets' for blends are. Without them it's very hard (unless you're working on a very small area) to get a decent blend going.
Now the wet part - paint the middle of your progression with the bronze, thinned a bit with glaze medium. This'll help keep it wet, and allow you to apply basic highlights over the washed surface.
Thin one of the mixed steps with a little glaze medium and work from that part of the progression toward the (still wet) bronze.
Repeat for the other side of the progression.
Dry this.
Now add a little more aluminum and black to the mixes for either side. Glaze those gently in (this is more layering than wet blending) at either side of the progression either to highlight the outer edges (smoothly, not an edge highlight) or shade the centres of areas at the ends.
Dry this.
Finally, edge highlight with the very light mix, and do the very extremes with pure aluminium. I also generally paint the chipping in the light and then colour that in with the very dark mix.
