Hello Dark Night,
Let me share some of my thoughts on what you have written.
Currently I'm sitting evenings only, and have been doing so for about six months now, but already feel myself beginning to falter...
Not sure how exactly you are faltering. Maintaining practice for six months is not a small thing. It is indeed laudable. Are you setting too high standards for yourself, like you should be sitting for hours, be able to focus continuously etc? If so, such things are going to put you in trouble. Relax, let go, you are doing good.
My problems seem to revolve around doubts as to whether I'm actually meditating "properly"...
Yes, you are correct. This is the main problem. You need to relax and let go. You are doing good.
...and if so, why I haven't experienced the positive changes in my life that Goenkaji speaks of!
Two things:
1. What Goenkaji talks about in his talks as benefits are only general examples. It is NOT necessary that everyone should get ALL benefits, and also not necessary that everyone should get benefits on similar time periods. There are always individual variations.
2. In your post, you have actually listed out many benefits, but somehow you seem to be discounting them, on the ground that they are not what you are actually looking for. You need to let go of your assumptions and expectations, and focus on the here and now.
I usually start with anapana until my mind quietens down a bit, then I work on vipassana body-scanning as Goenka instructed, finishing up with metta bhavana.
This is a good way. You are doing it correctly.
A fair amount of mind-wandering often occurs, but I'm generally able to bring my attention gently back to the body without irritation.
Mind-wandering is natural. Even advanced meditatiors experience this around 80% of the time.
I usually feel subtle sensations (with some "hazy" areas).
Pass through areas of subtle sensations and hazy areas separately. Wherever you find hazy, misty or blank areas, spent more time there, keeping your mind equanimous. Then, continue being aware of the whole body subtle sensations. Then, again pass awareness through hazy/misty/blank areas seperately. Cycle through like this continuously. Eventually, the blank areas will start opening up.
In fact, I'm aware of sensations a lot of the time when I'm not meditating...
Excellent!!! Continue doing this more often. This is practice off-cushion.
In addition, the body-scanning has become very automatic...
Do not allow the process to become automatic. Whenever you catch yourself being on automatic mode, immediately change the way you are moving your awareness through the body.
...and suspect that I'm not really meditating properly at all; just sitting around daydreaming.
No, you are not. You are doing great, but you are discounting lot of the things you are doing right, by comparing with an ideal, and by holding too high expectations. Relax, let go, observe.
What does seem to have happened is a profound shift in values and lifestyle - I have no idea whether this is a subconscious process, or simply a result of reading about Buddhism. I used to be a bit of a party animal and a big drinker (borderline alcoholic), also dabbling in recreational drugs. I was an "experience junky" (wanted to do anything and everything); had loads of mates and a full-on social life. But a year after the first retreat, I astounded myself by giving up drinking completely, along with a lot of other activities and plans (losing many friends in the process). Following my third retreat in 2009 I just stayed vegetarian, and even started avoiding dating.
You have mentioned so many positive changes here, but you seem to be disparaging them and I also suspect you have ignored so many other little benefits just because you don't consider them benefits at all and so, have not even mentioned them in your post.
What am I doing wrong?!
Nothing. You are mostly doing things right. You just have to relax and let go of thinking this whole meditative path like reaching a goal or something, by doing everything "correctly".
Is there, in fact, a "correct" way to meditate?
I don't think there is a THE RIGHT way to meditate. All instructions are guidelines to help you be fully present and realise the Three Characteristics (impermanence etc). Moreover, from what you have written, you are doing pretty OK.
Why have I experienced only negative results so far?
And by asking that, you have slighted all the positive benefits you have received so far. Some negative effects can occur when meditation is continued, but they are also impermanent. So, you should use your faculty of equanimity to deal with them too. They will also eventually pass away.
Do I need to maintain a continuous, intense daily practice for a year or two before I start to experience anything positive?
Drop the desire to experience positive things, attain perfection etc. All desires are hindrances to the path. Be present, observe all sensations, realize the Three Characteristics.