I've been using this set for a while and it's been pretty effective.
www.smogon.com/dp/pokemon/registeel
[SET]
name: Rest + Sleep Talk
move 1: Rest
move 2: Sleep Talk
move 3: Curse
move 4: Iron Head
item: Leftovers
nature: Careful
evs: 252 HP / 40 Atk / 216 SpD
[SET COMMENTS]
<p>With its Steel-typing and base 150 defenses, Registeel makes a very durable wall. However, Registeel lacks a reliable recovery move. This set remedies that problem by using the Rest and Sleep Talk combination, playing similarly to Crocune.</p>
<p>The objective of this set is to boost Registeel's mediocre Attack high enough to actually hurt opponents, while also boosting its stellar Defense to assist in walling. Once Registeel has Cursed 2-3 times or more, Rest off any residual damage and attempt to sweep. While Steel isn't generally a very effective attacking type, it does have the luxury of being able to damage every Pokemon except Shedinja; one such property the standard Crocune lacks. It is recommended, though, that you use Magnezone or Dugtrio with this to help defeat Steel and Water-types which completely wall this set. Registeel also defends well against special attacks. For example, it is 4HKOed by Timid maximum Special Attack Shaymin-S' Earth Power, which is Shaymin-S' best option against Registeel.</p>
<p>Maximizing HP EVs makes Registeel as durable as possible. A large amount of EVs are allotted to Special Defense to bolster Registeel's non-boosted defensive stat. The rest of the EVs can be distributed to Defense or Attack, to assist in walling or offense, respectively. Attack is preferred though, as Registeel's offense is pretty weak.</p>
www.smogon.com/dp/pokemon/registeel
[SET]
name: Rest + Sleep Talk
move 1: Rest
move 2: Sleep Talk
move 3: Curse
move 4: Iron Head
item: Leftovers
nature: Careful
evs: 252 HP / 40 Atk / 216 SpD
[SET COMMENTS]
<p>With its Steel-typing and base 150 defenses, Registeel makes a very durable wall. However, Registeel lacks a reliable recovery move. This set remedies that problem by using the Rest and Sleep Talk combination, playing similarly to Crocune.</p>
<p>The objective of this set is to boost Registeel's mediocre Attack high enough to actually hurt opponents, while also boosting its stellar Defense to assist in walling. Once Registeel has Cursed 2-3 times or more, Rest off any residual damage and attempt to sweep. While Steel isn't generally a very effective attacking type, it does have the luxury of being able to damage every Pokemon except Shedinja; one such property the standard Crocune lacks. It is recommended, though, that you use Magnezone or Dugtrio with this to help defeat Steel and Water-types which completely wall this set. Registeel also defends well against special attacks. For example, it is 4HKOed by Timid maximum Special Attack Shaymin-S' Earth Power, which is Shaymin-S' best option against Registeel.</p>
<p>Maximizing HP EVs makes Registeel as durable as possible. A large amount of EVs are allotted to Special Defense to bolster Registeel's non-boosted defensive stat. The rest of the EVs can be distributed to Defense or Attack, to assist in walling or offense, respectively. Attack is preferred though, as Registeel's offense is pretty weak.</p>