I think there may be a lot of pressure to increase the number of representatives if Rhode Island loses its 2nd seat or if Montana's population reaches 1 million. Also, if/when Puerto Rico becomes a state there would also likely be some sort of permanent increase in the size of the House.
There wasn't a permanent increase when Hawaii and Alaska became states. The number temporarily increased to 437 before going back down to 435 after the 1960 census... But I would hope that if PR or DC ever become states, they would just permanently increase the size accordingly. 
It went back to 435 automatically. The way the current apportionment law stands, if a State is admitted, the House automatically reverts to 435 after the next reapportionment.
Maybe that law would be overridden if Puerto Rico became a State, since going back down to 435 would require 6 or 7 other States to lose a Representative at the next reapportionment. DC won't become a State anytime soon, and even if it did, 1 State losing out won't cause a change in the way the law concerning the size of the House to be made. It's too dependent upon the Federal largess for the Democrats to be happy about it becoming a State in the usual manner, and too Democratic for the Republicans to agree to an amendment that would make DC a quasi-State still eligible to be treated differently than other States for the Federal largess.
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