I'd say its all about buying below market value , market conditions and rental yield.
When prices are about to fall, or are falling its generally best to offload(ie buy to sell) the property assuming its not an amazing yielder(ie much higher rent than mortgage cost with little to no management).
when markets are good to booming, keeping hold of property is key, its about holding as much asset value as possible coping with cashflow and refinancing as soon as there's equity re-investing. Its knowing when to slow down and stop / cope with negative cashflows / cope with time and managment of tenants/maintance thats important when markets turn. holding a high geared portfolio with break even or negative cashflow can be dangerous if this can't be rectified or if there aren't enough reserves to cope with this situation for some time.
credit score and ability to obtain mortgages is very important.
I've recently hooked up with a company that helps people out that have over commited on gearing - who have manily bought low yielding property and who want to consolidate their equity position and sell without having to pay CGT tax.
regards andrew Peers - andrewpfs@aol.com