I presently have 16Ft floor trusses joists spanning 16ft4in. They have been strengthened with plywood in certain places because they were put in backward. The fix it sheet was supplied by the truss company.
Its been 10 years now and would like to put ceramic tile on top for flooring. I think there is too much flex and bounce in the floor. They are 24in oc with 3/4 in TG plywood. What do I need to do before I put the tile down??

It's impossible to tell without knowing everything there is to know about the trusses, and the only people who would know all that are the manufacturers. The basic ways to reduce "bounce", which is different from "deflection" are to add mass, such as a 5/8-inch gypsum board ceiling below, or to make the structure stiffer, such as by adding additional trusses, probably not practical, or by adding material to the trusses to increase stiffness. The latter can only be determined by the manufacturer, or by a structural engineer after exhaustive calculations, which the manufacturer has already done.

Since you have 24" o/c spacing, you have to look at the subfloor.

What you do not want happening is the plywood seams/joints actually deflecting.

If this happens, you will get either grout crack or tile movement.

Adding another layer of subfloor or re-inforcing the subfloor joints from underneath will help.

If you have a problem with deflection... Maybe you want to consider another flooring besides tile. I'm afraid you'll fight it the whole time it's down.

Here's my two cents. The deflection is the issue, not "bounciness" - that's a dynamic problem.

Deflection can be helped by adding more trusses, but that will not help with vibrations. You want a really tight deflection limit if you're supporting a brittle material (like ceramic tile). That being said, the DL is already in place, so the only real deflection to concern yourself with is LL deflection.

Without knowing anything about the trusses, I would say that if you want to stiffen them then sheathe the sides. Apply plywood to both sides of the trusses and attach with a reasonable nailing pattern.

I will qualify this with a few statements. first, there are probably ducts running through them so that coule get tricky. Second, without sizing the nails or patern, the nails will eventually start making the holes bigger and reducing their effectiveness.