You can't increase the physical RAM, but you can use your internal or external SD card as RAM. Although this isn't something I recommend, especially not your internal SD. This is because RAM is designed to handle a lot of reading / writing to it, and your flash memory isn't. So if you use it as RAM, your SD cards life time will surely be decreased.
I'd recommend either to get a new phone, or just live with it.
There are some things you could do to preserve your RAM, for example greenify
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X10 Mini Pro (U20i)
Samsung Galaxy S (GT-I9000)