do not add extra fat to the prime pork I use, simply breadcrumbs and fresh herbs like basil or chives. I also keep the salt down. Just tried to make pork and onion and they taste crap...
Unfortunately, a certain amount of fat and salt are necessary evils to turn "mince" into a good sausage.
To add a bit more detail to johnfb's summary, which has worked well for me too:
All the advice on here gives 80%lean:20% fat as the recommended ratio for sausage making.
To the eyeball, I have found a ratio of 3 parts lean shoulder : 1 part belly pork to work well. Make that 4:1 if either is very fatty. The belly also seams to lend a soft, succulent texture and has a lot of gelatin.
Salt is necessary to get a good bind when mixing (as per Oddley's post above) 1.5% or about 1 level tbs /2lb (1Kg) is about the minimum.
The key to not having the sausage ooze out of the ends is in the mixing, and again after stuffing to leave enough time for the skins to bloom.
I add iced water at 10% of weight of meat , together with the 1.5% salt and seasoning mix (about 0.3%) before mixing . Mix until the meat has absorbed the water and goes from crumby to stringy. Put back in the freezer for about 1/2 hour. Then add rusk and mix well. Leave again in freezer, or bottom of the fridge while preparing casings.
NB regarding taste, if at this point you have fried up a test sample, it should taste bland as it takes a while for the herbs and spices to develop flavour. You can leave the mix covered in the fridge overnight to be sure. Don't go overboard adding more than the recipe calls for or you will regret it later
To avoid having excess water left in the casings after rinsing, I find "chasing" the water down the casing with a few drops of light olive oil helps ease the delicate sheep casings on to the horn.If any remains it will solidify when cold. Pat or squeeze the outside of the casing dry once on the horn.
As johnfb says, do not overstuff the casings. I restrain the feed just enough to fill out straight off the horn, without stretching the casing or "ballooning". If there is any slack in the casing it is taken up when you pinch out the links.
Once stuffed, link the sausages (pinch out and maintain a finger width between links) and hang overnight in the fridge to bloom. This allows the skins to relax and the twisted ends harden into place so the skins dont try to shrivel back sending forcemeat out into the pan. As the skins dry out they will cut or bite with more of a snap than a rubbery twang.
I hope this helps, I've learned a lot from this site over the past year. I too was in your state of dispair when I first came here. Today I just made a second batch of the Gloucester recipe as that scored a big hit with friends...Thanks again to all