Sight reading for Guitar can be a challenge. It is only certain genres of music that call for it principally classical, possibly some folk and some flamenco, then band and orchestral settings.
Some types of music benefit little from being notated (think of Jimi Hendrix) because much of what is being delivered either cannot be written (e.g. feedback bends), or would be highly complex to write down. That's why the guitar books of the high street sound so lame. They are more simple guides (and are often highly inaccurate.
This does not mean that sight reading is not a useful, even an essential skill. It can also teach you harmony ad theory, for example and its usually the best thing for jotting an idea down, along with tab.

If I were to learn it again, I would probably start by writing out my own chart for VERY simple songs, such as folk tunes - three blind mice, silent night, amazing grace etc.
This approach helps you realise what is going on, and after a few months gets easier and easier.

I have a problem with developing sight reading. I am fluent on a number of instruments, but on the Piano I memorise everything by habit, phase by phrase, this means after a couple of runs through a tune I am not sight reading, and I don't need the chart.

This is of course a good thing in many ways, but it also means I cant easily practice sight reading, as I need a flow of new material.
Many advanced 'classical players' fall over when they cant see the page. This can be a real handicap many cant improvise only read. A concert standard pianist does not need a manuscript except in the early stages.
If you want to develop your musical memory for charts, the key, the secret, to doing this is in fact simple. You set yourself the goal of memorising every phrase, when you first read it - not moving on until you can play the phrase without looking at the music too, then you treat the next phrase the same way and and then marry them. This may seem a huge burden and unnecessary at first, but if it is built in to your reading style right way from day 1 with no exceptions, then after a few months its habit. Lastly, good readers read ahead of the notes they are playing, they scan the notes of the next few bars and prepare themselves for the difficult bits, whilst playing the current bars. That skill really helps fluency and is twinned with the memorisation skill.

Of course this is only my take, but you might want to pick bits out of it.

Z

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