How would you post a journal entry when you received the cash in advance for a service to be done in the middle of the month but don't complete the services by the last day of the month? Would you have to do anything? Or would you just wait until the work is done to enter it?

To record the Cash received the Journal Entry is as follows:

Debit Cash Received for the Amount Received
Credit Unearned Service Revenue for the Amount (This is a Liability Account)

To Record the partial amount of work done the Journal Entry is as follows:

Debit Unearned Service Revenue for the amount of Service Performed
Credit Service Revenue for the amount

Example is for Advertising:

Received $500 to perform Advertising

Debit Cash for 500
Credit Unearned Advertising Revenue for 500

After Partial period of the Advertising work Performed:

Debit Unearned Advertising Revenue for 200 (Amount of Advertising performed)
Credit Advertising Revenue for 200 (Amount of Revenue Earned)

Note: Still $300 of Advertising work still to be formed (a Liability).