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Every time I try to make a file Loopable I get a break or transition where the file ends. When I try I cut a little but it's still that way. I end up cutting a little farther and so on until my file is totally cut away.

Take a look at this video of mine,
https://www.youtube.com/watch?v=hiwC05zMaFw

especially where it mentions the zero points; the non-zero start and/or end cause the clicks you hear.

If there are no feasible zero points close enough to the loop size you need, try applying micro-fades at the start and end of the loop.

By the way, many audio players offer looping options, but not all are capable of sounding perfectly seamless; trust Audacity for that.

If your sound has a noticeable beat to it, you have to align the cut points to stay in sync, like right before the most noticeable and striking beat, preserving any measure boundaries, etc. So that part is obvious. But what might escape your realization about that is that this also holds true for perceptual modulations within the sound -- those also need to be aligned. So if there are any filter-sweeps, chorus or flange effects, etc., then it can be impossible to get them all to align unless you have planned ahead and made sure that they all have a common integral multiple of their period.

For example, let's say there is no percussive element to worry about, but you still have a chorus on one component of the sound, and a tremolo type of effect on another part, so you pick frequencies of 0.25 Hz one for a period of 4 seconds and a frequency of 0.2 Hz on the other for a period of 5 seconds -- these will both sync up every 20 seconds. You could pick 20, 40, 60, etc., but make sure you have an integral number of periods for all modulations.

So you then try to set your cut points for beginning and ending at as close to that selected period (say, 20 seconds) as you can get, but you still need to get the waveform as close to a zero crossing at the cut point as possible. This can also be quite difficult if dealing with a multi-channel sound. You may have to do some micro-editing (such as a very short fade out and fade in across less than a single period of the lowest-frequency present, but that can be a bit challenging to do without making it sound bad) to get one channel to be forced to zero-cross where the other does, still near that 20-second total period. You can also try applying a phase-shifting filter to just a region around the cut point on the channel to see if you can force it to have a zero-crossing in the exact same place (obviously, you have to fade the effect of that filter in and out across a segment around that cut point).

Once you've gotten your zero-crossing cut-points for the beginning and ending at the right distance apart, you still are not done. You don't just crop the sound to that 20 seconds and call it done. If there is any reverberation or delay, you'll need to keep some of the tail after the cut point, depending on how long the reverb remains discernable over the dry sound, perhaps 1/2 seconds worth, or maybe even 10 seconds.

Let's say you pick 2 seconds worth. So you trim (crop) the whole sound down to just the 20-second period you selected plus 2 seconds of extra after that ending cut point. Now you select everything from that final cut point to the end. Let that two seconds of post-cut point sound do a linear fade-out to nothing. Then cut out that segment out, keeping it in your cut buffer. You'll need to select the exact same amount of waveform from the beginning to do a corresponding fade-in, but for me, I find that in Cool Edit Pro (probably also Audacity) it is easiest to just go head and mix-paste it over the beginning just to auto-select the exact segment it will overlap, and then immediately undo that mix-paste operation, which leaves that segment still selected. The idea is to know the exact point where that wave will overlap the beginning. You could also just measure it (down to a percent of the total length would be good enough resolution, but the mix-paste/undo trick gets it exact to the number of the samples). So then you take that segment without anything overlapping it yet, and do a linear fade in from nothing. Now you're ready to mix-paste the fade out that you cut earlier over the beginning of your cropped sound. Note: your fade-out and fade-in need to compliment each other, so either a linear shape of a half-cosine shaped window would work, but don't use a logarithmic taper that is at some level other than 50% at mid-fade, or the levels won't stay constant.

And the truth is, sometimes you have some destructive interference for certain frequency components between the original sound at the start and the overlapped sound that you mix-paste over it. That happens because the same frequency may be present in both parts but it is nearly or exactly 180-degrees out of phase (i.e., it is inverted). You usually notice it most in the most dominant frequencies present, which also happen to provide most of the magnitude of the waveform as you look at it in your editor. So you can try to compensate for any dip you see at the mid-point of that cross-fade by selecting that section that dips and apply an envelope to it that boosts it enough (keep the envelope at 100% or 0dB at the edges, but higher in the middle) and with corresponding placement of the peak of the envelope to match the dip, so that the result looks pretty stable in overall volume, or sounds right by ear. Unfortunately, you may have to apply different amounts of boost to different frequency bands in order to get it to sound okay, and how to do that depends a lot on your editor. But generally speaking, many sounds will not have much of this kind of problem at all, so you may not even face it at all. Regardless...

The result will be a seamless loop that has a zero crossing at the start and end.

However, if you save it to a format using lossy compression, all that effort will be for naught. You really must have to a lossless format so that the zero-crossings are properly preserved.

Well, anyway, that's how I usually do it. Hope that helps.

Oh yeah, another gotcha to watch out for could be a feature that your editor might be providing that does a "smoothing" of edits around edit boundaries. That kind of nonsense is not wanted when you are trying to control things down to the exact sample. So make sure any of that is turned off for your edits around the cut points.