goochie
this is my advise, some of which i must credit to this forum, this is what i do, firstly i would use DOWEX resin (i've tested it against purolite & it lasts far longer).
secondly, when your tds starts to rise above 000 (but dont pannic as 001/2/3 is still ok if your caught out during the day), take di vessel out of van, put a stop connector on each end & give the whole thing a really good shake up/down, side to side etc for a good minute or two. the objective here is to mix the resin up inside. after this let the vessel settle for 10 mins or so, run a couple of litres of water through it & hey presto!! back to 000tds. repeat this when tds starts to rise again but remember that each time you do this, the periods of 000tds will get shorter so frequent testing is advised untill you get used to it. obviousley the resin will eventually expire totaly & this is when the above no longer works & you will have to change to new resin.this is when you swap the vessels round.
As the water flows through the vessel it creates channels in the resin so only part of the resin is actually working ( the bit which the water flows through). by doing the mix-up thing, this changes that & so the flow starts to make a new channel through now fresher resin & so forth & so forth.
hope you get all that, its amazing how good this works & physically shaking the vessels IMO gives better results than backwashing them (just encase someone else posts to confuse things).
tony