The shmone esrei is divided into three distinct section: three paragraphs of praise (shevach), 13 of requests (bakashos), and a final three or thanks/acknowlegdement (ho'da'ah). (The observant reader will note that adds up to 19, not 18.) Each section has its unique character that expresses itself in halachic distinctions. The shevach and ho'da'ah are inseparable units that must be said correctly from start to finish. The bakashos need to be said in order, but to correct a mistake in the middle that requires a repetition requires only going back to the bakashah in which the mistake occurred. One may make additions for personal needs in the bakashos, but no personal additions may be made in the shevach nor ho'da'ah. There is a very nice distinction that shows up in the change we make between winter and summer that spans both shevach and backashos. In the winter: "mashiv ha'ru'ach u'morid ha'gashem" in shevach, "v'sein ta...
This is a paraphrase of the pasuk in t'hillim 84:7 -- "mei'chayil el chayil" -- which means "from strength to strength". In this case, it is my thoughts and ideas to those who are strong enough to be interested :)