I filed my taxes on 4/4 and I still do not have my refund. The status is in processing. What can I do to receive it faster or get a response from the IRS?

No refund after 3 months of waiting. After filing, I received a letter requesting more information for proof of income. I sent this back in via certified mail. I have received no further communication from the IRS. Every time I check, it's simply in "processing" status. What can I do to expedite my return?