my tip is change some cash before arriving overseas. Generally (except for singapore) you will get a better rate selling at "home". ie, buying GBP in Aussie with AUD is better than doing the same in London by quite a margin. Just find a place in the city etc that doesn't charge commission. If you use ATMs that charge an access fee and withdrawing 1-200 each time, the fees percentage wise are horrendous...5-10%. Circumstances will dictate what you do.
Best deal by far for cash is available for
AMEX card holders.
At AMEX booths in Westpacs they charge the same rate for cash as for travellers cheques (so save 1-2.5% vs other cash providers). This is available for anyone - not just AMEX card holders.
Where the real benefit springs is that you can BUY your FX cash using AMEX REWARD points. Up to early 2011 it was 100 points could be redeemed for $1.00 - so best rewards point benefit going. In 2012 they changed the rate to 120 (I think it was when I last used it) for $1.00 - still pretty good value vs other reward point uses.
Also, if you want to be a real miser, just wait around a couple of minutes and if you see someone about to go up to them holding the FX you want - then deal direct!
I stumbled onto this one day while sitting down to work out exactly what split I'd need between Euros and GBP and heard the unmistakeable British accent from a couple saying 'at last we've found a place to change our pounds".
Being a shy type (until there's money involved) I immediately stood up and said I was about to buy some pounds notes so let's cut out the middle man and I'll pay you more than they will. Long story short I got GBP 2,500, they got AUD80 more than AMEX were going to pay and I saved AUD 120ish (crossing the buy/sell spread allowed me to give them some benefit) and we both saved the $8 fee.
Tried it the next year, thought I'd read the paper for 20 minutes one lunchtime (late Dec) and sure enough heard some French people coming in, saw the Euros being assembled and made them an offer. Got nearly 3,000 Euros etc.
Just depends how lucky you are on the day (and time of year I suspect). Mid to late Dec seems a golden period.