IASbet offer a 10% deposit bonus today only,
but on the other hand new memebers get 20%,

So which makes more sense, top up the old account or open a new one??

Logic tells me that 20% is better than 10%! But I'm pretty sure if the 20% is for new members only you wouldn't be able to open a new account in the same name as your current account.

Open a new account. Then top it up.... lol
Top up your other one.
Open one in your mothers name, your fathers name, so on, so on....
Then top them all up....

Does anyone know if IASbet does this kind of top up promotion regularly?

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...time held me green and dying, though I sang in my chains like the sea. - Dylan Thomas

Not sure about the existing member deals but the new member bonus has been going for a while now. Sort of lousy new members get better treatment then good standing existing members, you'd think they would treat the current customers better.
Well like somebody else said if the account runs dry open a new account for mom, dad, sis, aunty flo, the dog . . .