Okay, I'm a beginner too, but at least my plants are healthy. For what it's worth:
Mother in Law's Tongues can stand incredible amounts of abuse, but perhaps it depends on the kind of abuse. And you can kill them with kindness. Important: what form is your MILT's slow death taking? Rotting at the base? Wilting leaves? Pest infestation? So: the temperature's not constantly chilly during the winter? (Heaven forbid letting it freeze.) The light level's not pitch darkness or anything near? The soil isn't heavy? I have mine in light soil, in the kitchen (not near a window, so meager indoor light), and water them about once a month, which seems to be about right. If temperature and light are within acceptable limits, then if they're rotting, I would guess too much water; if the leaves are wrinkling up, too little. They can get by with poor levels of nutrients, I believe, but the sansevieria nurseryman I asked told me that slow release fertilizer is good, and actually a good deal of water in the summer. The classic MILT likes light in summer when it's warm, but filtered, not direct: I have mine outdoors under a wisteria pergola, where they do well. I would think that a too-large pot wouldn't kill one, though it might not grow as you would like in that case: it might be healthy but not put out a lot of new shoots. Maybe.
Robin: does any of this offer a clue as to your problem? Good luck solving it! I'm also interested in what others who know their stuff about sansevierias think of my comments. Feedback would be appreciated, as I'm just starting to learn about these plants as well.
Melissa

Robin

Can you provide a little more info for us? How big is the pot its in? It needs a tight pot-they love to be rootbound, almost busting out-and something well draining like clay. Heavy clay pots will also prevent the plant as is gets older from toppling over. It can be easily overwatered in a large pot. They are succulents & can take no water for months, but watering every 3-4 weeks is normal. It depends on the light and temps it is recieving. They can take a good amt. of sun-mine get about 5 hrs of direct sun a day-east facing is fine too.

Melissa, your advice is very good! I bet your Sans look wonderful. Happy growing, and I wish you lots of luck Robin!

Jen

Well, from the replies from both of you it sounds as if my problems are: way too large of a pot, soil too heavy and packed and too much water. I will repot in a much smaller pot so it can become rootbound, with fresh potting soil and refrain from watering.

Thanks so much for the help. At least I have a direction to head in now!

I'd agree w/ repotting it much smaller, preferably in a clay pot (w/ drainage hole) & go easy w/ the water.

Just as insurance, you might want to take off a leaf or two & use those to start a new plant or 2, just in case.

Just so you know, overwatering is THE one sure way to kill a Sans. This can also happen by using too large a pot (even if one doesn't water all that often). The reason is this, the too large pot keeps the soil too wet, too long; longer than the plant can take up the water from its roots & ultimately it will start to rot, or just collapse altogether.

Jen: thanks for the feedback and kind words!

Robin: good luck!

I much appreciate the general information on cultivation in this thread.

Back w/ a PS:

I must pls. respectfully disgree w/ TJSAngel. I keep my Sans. indoors all year & most of mine get watered at least every 2 wks, some every wk. I think the suggestion above of water every 3-4 wks, is too little water (esp. in Summer).

Anoher PS, I recently learned the hard way: the shorter type Sans. (commonly called 'Birdnest' types, more formally called Hahniis (sp?)) can be much more sensitive to overwatering & can die quite fast, esp. if one gets water in their crown (I just lost one from this).

Otherwise, from the 10 or 12 diff. kinds I grow, I don't lose them very often.