What are the best settings when converting, say, a .mp4 video to WebM?

I have done some searching on here but nothing specific to what I need really.

What I've been using:

avconv -i input.mp4 \ -c:v libvpx -qmin 10 -qmax 42 -maxrate 500k -bufsize 1000k -threads 2 \ -c:a libvorbis output.webm

However this seems to have issues. I can not upload/embed it on sites that support WebM and if I get it to it sometimes will not have any video, just audio. I have also tried without the qmin, qmax, maxrate and bufsize options or a combination of them. I have also tried using AviDemux with the same issue. Says I can't upload it on certain sites or ones I can it will have playback issues.

I'm not great with video codecs and converting so sorry if I'm just missing something really obvious. Any advice would help. Thanks in advance.