Generally speaking I recommend buying investment properties where the rental income is twice or better of the mortgage payment (P+I), excluding taxes, HOA, insurance, management fees, etc. Premium areas will require some flexibility to this rule.
For example, if you purchase an investment property in South OC with 30% down and monthly payment (P+I) of $1,500, ideally you'd want the monthly rental income to be $3,000 or better. But because South OC is premium-ish, I'd accept $2,800/month rental income.
You need the cash-flow to pay for the expenses (taxes, insurance, management fees, HOA fees, repairs, etc) and reserve fund, so when you're between tenants or selling the property, you won't be stressed out from lack of rental income during those months. If you have a healthy reserve fund you can afford to be choosy with tenants or buyers -- less stress. In contrast if you're cash flow negative, you'd be pressed to accept less desirable tenants or low ball offers while the property sits empty.
However, just because the numbers look great doesn't mean it's a good buy. I have 3 friends who bought investment properties in Vegas and they all got burned one way or another. Finding good property manager was a pain, finding good tenants was a pain, tenant leaves behind trail of destruction and trying to find a good contractor who won't bail halfway done with the job was a pain, etc. etc.