check the technical spec first to understand what the report is doing. this will help you to chalk out which areas can be optimized. if you dont do this you will spend too much time reading code and this will spoil your day.
avoid repititive selects on same table as much as possible. make sure all the selects are outside of loops. try to select data at one go for a table. if selects are slow try creating table indexes to make selects faster.
n.b.... avoiding CORRESPONDING FIELDS OF doesnt help.. in fact using it is better. I have read a blog on this... sap handles things better than us so you can use it safely.
try using field symbols .. use sorted internal tables if possible... specially the ones you will search into
most performance issues can be solved by non repititive calls to function modules / table selects which can be done in one go..