Try and get experience before, during or/and after your undergraduate degree. The more clinical work experience you get the better. You need to try and build up a portfolio, e.g. support worker for people with learning disabilities, working with patients with dementia, geriatric psychiatry, schizophrenic hostels, residential homes, nursing homes, volunteering in psychiatric hospital wards etc. Obviously, in an ideal world you would be able to get a job as an assistant psychologist for the NHS or something similar. From what I've discovered these jobs are rare to find and are highly competitive.
On top of all this, you need to come out the best classification of degree you can gain, so you would be aiming for a 2:1 or 1:1 (Unless your incredibly gifted, this would take a lot of hard work and commitment) Clinical Psychology is very competitive and difficult career to break into. After I dropped out of University a year and a half ago, I started working as a Care Assistant in a hospital on trauma a ward, I'm now doing this along side working in a Neuropsychiatric ward (On the nursing bank), this is my second year working full time. In September I'm going to study my undergraduate degree in Psychology.. I'm still looking at Six years minimum before I can call myself a qualified clinical psychologist. (And thats assuming I gain part time relevant clinical work throughout my degree... balancing academic studies with part time work is never easy)
Its definately a long winded road... not for the fait hearted.