One of the less provable aspects of this debate between AV or FPTP is this:
Which result will better enable people to call for further reform?
To me it's a redundant question, but let's look at some realities we do know.
1) The No campaign is mainly organised by Tories and Labour peeps that actually don't want to move away from FPTP. There are those involved that are honestly of the belief that a system that means something like 25% of the electorate could stay at home without affecting the 1-2 placings of every constituency, is better than one that gives everyone a more equal voice. There are also a certain amount of those who believe that accepting a small change will put a barrier up against bigger change.
2) The Yes campaign is mainly made up of Lib Dems and Labour peeps that see there is a democratic deficit in this country that needs resolving somehow (though, I think it's fair to say both sides tend to disagree on just how that should be solved)
3) Referendums kill other referendums. In the 70s we had a referendum on the EU (which was won as a Yes vote). Labour recently faced calls for a repeat of this referendum under different times. In the 70s a major push was made by the Tories to go in to the EU for economic reasons, in the 00s the popular rhetoric was one that we had "had enough". As such Labour basically said there would be no referendum because it hadn't been long enough since the last one. Political posturing based on knowing the result most likely wouldn't have gone their way. (I support being in the EU, for those who care).
So what does this all mean? First, if you vote no please don't pretend that you're not giving a victory to those that want to ultimately keep FPTP. If you vote yes, don't pretend that there aren't No's that don't want reform..or that you're voting for further reform. You're voting for AV, that is all.
Also, the fact this referendum is here means that another referendum will not come about without a party political wish for there to be one, regardless of the result.
But then how the whole "further referendum in the future" thing goes is anyone's guess. If the public truly make a stink about reform...protests, mass petition/engagement with politicians...then maybe it'll happen sooner as whichever party in power thinks they can gain popularity off of it. This will happen regardless of whether we vote yes or no, the power is in our hands at any time to collectively try and force this.
The only other situation is that someone like the Lib Dems does actually get the main stay of power (at least the reverse of the current coalition), with party policy that they pursue STV and as such are mandated by their members to put a referendum on the table as a party of government. Of course, I think we all understand how unlikely this is!
So given this, how should we vote? Obviously it depends how you feel, AV or FPTP being better (despite it being demonstrably evident that AV is an improved and fairer system than FPTP), but Game theory for me says this...
1) The referendum is Yes, and we get further reform referendum after = PR people have what they want
2) The referendum is Yes, and we don't get another referendum any time soon = No proportional reform, but a better system than we had for the time being
3) The referendum is No, and we get a further reform referendum after = PR people have what they want...though the fall back if they lose again is still FPTP
4 The referendum is No, and we don't get another referendum any time soon = No proportional reform, and stuck with the same unrepresentative system.
Total it up, it's clear that you're more likely to get a positive result, not only now but if everything goes wrong in a future referendum, if you vote for AV now. A future referendum is not dependant on how this referendum goes, but how much we want it regardless of the result...the question now is what do we want the default position to be should that reform not come, or should that future reform fail to get supported by the public?