I think if you ask someone how to pronounce it, they will do a full unvoiced pronunciation of ths. Merriam Webster backs me up: http://www.merriam-webster.com/audio.php?file=month002&word=months&text=%5C%3Cspan%20class%3D%22unicode%22%3Eˈ%3C%2Fspan%3Emən(t)s%2C%20%3Cspan%20class%3D%22unicode%22%3Eˈ%3C%2Fspan%3Emən(t)ths%5C However, as to how we say it normally, I think you're right that it's not so clear. I don't know how to explain it linguistically, but I think we say mon and then the s with our tongue in the unvoiced th position. It's kind of a lesser th, but I think there's still a trace of it there. Does that help?

If you think of the word "month" in singular ends in [θ], but if you have to pronounce it in plural it is not possible to make this sound and then pronounce the "s", so, what you do is put your tongue in the position to pronounce the [θ] but actually pronounce a ð (in other words like θ but without letting any air out). and then pronounce the "s" sound. The same happens with the word "mouth" in plural... "mouths".

How do you pronounce an unvoiced "th" followed by an [s] sound? You simply have to practise this. If you try to substitute or omit the sounds, then you are saying it wrong. Start with your tongue in the "th" position, then let it slide back into the "s" position.

Months pronounce mons, the n link to s

Like " Monts " i think in American Accent :)