How can I tighten my kitchen sink faucet?

What ye seek be known as a "basin nut wrench". Holler at member "Dunecrazy" here.
Liquid conveyance mechanic of the first degree.

Here's the one I had when I did maintenance,there's 2 models. Home Depot carries them.
http://www.globalindustrial.com/p/to....campaignId=WU

Channel locks work for me. Just did this in fact.

Try grabbing the stem from the top (above the countertop) with your hand and twisting it. I had a situation like this in my last house where there was zero space up there from underneath and I screwed around with it forever, and couldn't get anything on it to get it tight. Until I grabbed it from the top and twisted it and it tightened right up, tighter than it had ever been, even when new. Of course my faucet had a section that I could grab and turn from above (the lower section of the faucet), like the image below. In other words, I could keep the faucet pointing straight forward, while I was twisting the lower portion of the faucet. Some/most faucets aren't made that way.

https://encrypted-tbn0.gstatic.com/i...rnaXfJJmACOnHw

I only use 90 channel locks. But the basin wrench is the official tool

Buff. Cut the copper above the fitting and pinch it then sweat it shut.