So, you want to shrink your turtleneck, but not so much it becomes a wardrobe addition for your miniature Schnauzer. First step: Consider the fabric.
Cotton and wool will shrink pretty easily. The hotter your settings in the washer and dryer, the more shrinkage you can expect. You can even shrink an article of clothing down a few sizes in just one wash-dry cycle. If you want to shrink those items just a small amount, check their size at various points during the wash and dry cycles. Also, consider starting the dry cycle on medium rather than high heat to mitigate over-shrinkage. When the clothing is just right, remove it. If the newly-shrunken item is still wet, then continue to dry it with the air-dry setting on the dryer.
Some polyester will shrink, but it may take a few laundry cycles for the drama to really unfold. Polyester holds up to heat well, so feel free to go for the hottest settings on your washer and dryer. Wash and dry the polyester items over and over until they shrink -- or until you call "uncle."
Still too big? Try applying a hot iron to the garment when it's damp. Be careful not to stretch the fabric; just heat it. When you're done ironing, toss the item back into a hot dryer.
Depending on the fabric, you can take your clothes down a size, or two, or even three!