Now is the best time to consider trading in your Galaxy S3 if you are bent on getting Samsung's Galaxy S4 once it hits global store shelves on April.

Everyone knows this is S4 launch week and if the estimates furnished by popular gadget trading sites like Gazelle.com and NextWorth.com are correct, the S3 you bought last year just lost 10 per cent of its current selling value.

Citing the two sites as sources, Gotta Be Mobile noted that by next month, which is the scheduled release date for the Galaxy S4, the outgoing Samsung flagship will further shed 15 per cent of its asking price.

So better shop for a deal that suits you on the two sites now, GBM advised. If you're the type that keeps a backup phone, the task is easier. You choose a deal, close it and ship out the handset.

While you wait for the S4's arrival, you stay connected with a feature phone of your choice or maybe an entry-level smartphone from Nokia or Samsung.

But without a second handset, a phone trade in could pose some problems. Good thing Gazelle and NextWorth both offer arrangements known as 'price lock', which allows seller to zero in on a deal price and enjoy the same price level within a predetermined period.

For Gazelle, the 'privilege' lasts for 30 days while NextWorth is a 21-day window. Say you made the deal now, between today and the first week of April, you can still enjoy your S3 before giving it up a few days before the S4's commercial debut.

There is no definite mark on how much you can still get for your old S3 but remember that pre-owned buyers give premium to handsets that look and behave pretty much the same when they were first unboxed.

Needless to say, if your unit is scratch free you'll get to fetch a high asking price. The better your prospect would be if the smartphone will come in the complete package - the box intact and all the accessories still functioning with a charm.

A realistic haul is around 50 per cent of the money you originally paid for the fresh Galaxy S3 buy last year, that is, of course, if you get lucky enough.

Regardless, what you'll snap up will surely lighten up the cash load required in grabbing the obviously more powerful Galaxy S4.

And one last thing, don't forget to 'factory reset' that old S3 of yours and wipe out the contents of its microSD card. This is for your privacy protection.