Step 1: Need for speed

As it would happen, there isn't much to do to make this happen. First, we create a constant, just to make our lives easier.

Snake.spin

    DOWN  = 2
    LEFT  = 3

    MAX_LENGTH = 256

    SPEED = 2

OBJ
    lcd  : "LameLCD"

Now we need to replace every instance of ++ and -- with += SPEED and -= SPEED.

These are shorter ways of writing certain operations that make your code easier to read.

x += y    ===>    x := x + y
x -= y    ===>    x := x - y
x *= y    ===>    x := x * y
x /= y    ===>    x := x / y

Here's what your movement code should look like now.

Snake.spin

            if ctrl.Right
                snakedir := RIGHT

        if snakedir == LEFT and snakex[0] > 0
            snakex[0] -= SPEED
        if snakedir == RIGHT and snakex[0] < constant(128-2)
            snakex[0] += SPEED
        if snakedir == UP and snakey[0] > 0
            snakey[0] -= SPEED
        if snakedir == DOWN and snakey[0] < constant(64-2)
            snakey[0] += SPEED

        gfx.Sprite(@dot_gfx, snakex[0], snakey[0], 0)

Ahh! Now it goes way, way fast at the beginning! I can hardly even see it. We can fix that, though.