based upon your english, it seems you want to convey a wish rather than a specific order. therefore a subjunctive of desire would be better than an imperative.
concisely you could say 'sis semper!' or 'sis perpetuus'.
if you wanted to use an adverbial form of perpet-, you would want perpetuo. you could say perpetuo vivas! which i think flows nicely (it's a choriamb and a spondee prosodically). you could of course use a form of litotes by saying 'may you never die', but that's up to you.