I was a bit...smashed, shall we say, when I replied to that and should have just stuck to the Liber MMM material. Apologies if I introduced any confusion with that, and I also understand completely why you would want to stick to the original material.
Anyway, back to the task at hand...
I usually concentrate on the anapana spot, where the breath comes in and out. should i drop that and stay just with the stomach?
Use whatever works best for you and allows you to maintain the focus in as stable a manner as possible; the anapana spot is good for a one-pointed focus, the style of concentration more common in magick and yoga, whereas in my experience using the stomach allows for a wider focus, but more stability as there's a larger surface area to work with. Practice your breath control
constantly, you don't need to be formally sitting to work on this. Anywhere you get an opportunity to stop moving and focus on the breath is an opportunity to practice; for example, I used to do this every day on the train to work, 30 solid minutes, twice a day, of informal concentration practice on the breath bumped by concentration skills way up.
I also can't seem to avoid some muscular strain when the breath is extended. Is this to be expected?
To some extent, yes. Are you fully expanding your abdomen, chest and shoulders with the entirety of the breath? Sometimes that strain can be exacerbated by not breathing properly; what's described in MMM is a basic pranayama-style breath, so maybe try adopting more of a yogic-breathing approach and watch the air to come into the body in three stages: fill the abdomen, but not to the point of straining, then the chest but keep the shoulders still while allowing the upper chest to expand comfortably. There's a knack to it, it's hard to describe but quite easy to learn.
Lastically, I suck at this. I've been meditating regularly for a year or two and am very underconfident in my ability to keep myself focused on that damned breath without dropping off just once in a )F^)&)&$ half hour. Maybe all I'm looking for are encouraging noises, but if there is some magical advice, i'd appreciate it.
If you don't mind me asking, what is it that you're looking to achieve through this?
Have you tried using sigils to state your intent?
A big part of seriously studying Liber MMM is deconstructing that lack of confidence and the beliefs you hold which prevent you from manifesting Kia. One of the greatest tricks to be learned in the study of magick is this: Fake it till you make it. Believe that you're a strong, skilled and confident meditator, eventually reality will reorganize itself to fit this belief (to anyone else reading this, I'm talking in Chaote terms here so don't misunderstand this as being a universal statement).
Any better?