I'm having trouble going down to fifth position from a higher position. I often land a little offkey (usually flat by a quarter-step unless I overcompensate), I think because that's where the base of the neck changes shape. It doesn't matter how big or small the shift is, and it happens regardless of whether I'm practicing scales and arpeggios or trying to play a composition. What's really frustrating is that I don't know how to fix it in practice. Can someone suggest a way of practicing this out that won't leave me with a slow glissando every time I need to shift down to fifth?
I practice shifting exercises as part of my warm up routine. This helps with pitch accuracy. They go something like this on my double bass:
A - long tone glissando to A one octave up. Gliss back down to A. Then Bb to Bb, B to B, C to C, etc.
A to A, Bb to A, B to A, C to A, C# to A, D to A, etc.
On violin, a better note to start with might be F# on the E string, or a B on the A string.
Increase the speed of your glissandos over time, until the gissando is imperceptible to simply playing one note from another. This should be gradual - always strive for slow and accurate improvement over jumping the gun and sacrificing accuracy just to gain some short-term speed improvements.
Next, I'll target specific difficult passages. Like the one you mention above. I will isolate just the shift, in the context of my scales, or more importantly in the context of the piece of music I'm trying to learn. I will practice just the shift, over and over, dozens of times. I will practice it slowly, as slow as it takes to get it perfect, then increase tempo with my metronome very gradually. Never compromise a perfect shift and intonation for a faster tempo, or else you're just practicing making mistakes. I will gradually expand the scope of the excerpt to include more notes before and after, so I can practice 'getting into the shift.'
Sometimes I'll record the target pitches and play it back while I practice to it. Or I'll set a drone pitch to tune to.
Technique-wise, make sure you maintain contact with the string and the neck. Don't your fingers off the string when shifting. The shift is a very tactile, very sense-of-touch oriented movement.
I haven't played violin for a long time, but as a beginner I used to have a narrow piece of electrical tape on the neck, marking (I think) the third. It came off once I had gained a feel for the position.
Perhaps you could try something similar. Mark the 5th -- it needn't be dead on, just a reference point. Practice pieces and exercises in which you're having this problem. Concentrate on how it feels, physically, to move to the 5th correctly.
When you can do it correctly every time, using the tape for reference, make a point of looking away when downshifting. When you can find the 5th without looking all of the time, the tape can come off for good.
Your thumb should always be your point of reference in whatever position you're in (yes, even if your thumb is on the right side of the fingerboard). Play a C on your E with your 1, ensure your thumb is placed properly and make note of it. Then try drilling the following while focusing on getting your thumb to the right spot, and then your other fingers:
As you drill these, you should start developing a natural frame of reference based on your thumb and index finger, and the rest of your fingers should fall in place :)
Note: it's okay to gliss a bit at first, but by the end of your drilling it should be completely clean.