Well one thing that generally works wonders in improving opponent attitudes is having a bigger space navy. If you have decent technology, not necessarily the highest, and the biggest fleets, everyone will be "friendlier"... 
In the case of Altarians, having a respectable diplomatic reputation, and a bigger fleet should combine well to keep them neighborly. But there's more to do if you want an alliance: attack their enemy if they ask for help, establish a trade route between your home world and theirs, and throw some weapon tech their way - AI civilizations love weapon technology more than anything! 
Beware that other civs, and random events, can and will undermine your relations with the Altarians over the long run, so don't take too long to make them your buddies once the window of opportunity is there.
