I would use sin/cos that is multiplied by a decreasing energy value "e". Additionally the sinus index "a" may increase/loop faster and faster (like in a rubber relaxing effect).
aspeed#=1.0 e#=2.0 while e>0.1 a#=(a + aspeed) mod 359 aspeed=aspeed*1.01 e=e*0.975 positionentity cube,0,sin(a)*e,0 renderworld() flip wend