Whats the best way to book a holiday to Dis World, is it better to book flights and accomodation separately? also is it better to get day passes to the park? We (2 adults & 3 children) are going next March/April.
Any specific tips please.

Well, Tom, it just all depends. Prices are adjusted very often so you'll just have to do some checking.
By the way, there's some really good advice contained in previous questions and answers.
In the TEXT SEARCH box, type in:
Disney
You'll have to ignore those that apply to DisneyLAND but people often use the words Disney, Disneyland and Disney World, and other terms that make a narrow search not worthwhile.
When you do this, look especially at #32; there's some really good advice about whether or not you want to stay ON Disney property or rent a home for your lodging.
Some people do not want to rent a car, bother driving back and forth, parking, etc. Others really like to rent a private home which gives Mom and Dad AND the kids PRIVACY, much more space than a hotel room and the ability to have a full kitchen, washer, dryer, pool, etc. There are many websites offering condos and houses for vacation rental; www.vrbo.com is just one. I think we'll do that next visit.
You'll have a great time no matter what your choices are.

Planning a trip to Disney World seems to be akin to planning a major military operation... and there are a lot of hidden traps or missed opportunities for the unwary. Strongly recommend you visit these two sites: http://mousecodes.com, which details the "magic codes" you can quote when ringing Disney to book accommodation at rates which the public never gets to see (not so long ago the All-Star resorts were available for $49 per room simply by ringing Disney and quoting a number from this site), and www.orlandotripplanner.com/wwwboard/, which is a question/answer board where, no matter how basic or involved your question is, you will find that someone else has asked it, and has been well-advised by experts who know Disney but are not employed by Disney. These sites will probably point you to still more; there are a lot of people out there with tips to share! (I recall reading one site with detailed directions for getting into the world FREE, right down to where to find the hole in the fence! I didn't try that one... I did, however, get a room at the Braodway All-Star for my family for $59 a night. This seems to be a better deal than the "cheaper-on-the-face-of-it" options of staying in one of the $30 motels outside the world and spending a good part of your day in traffic lines or parking lots... but, my own personal tip is that if you are staying on Disney property, take food in with you when you go, as buying even a simple snack at the resorts can break your trip budget in no time!)

www.disboards.com provides a LOT of great info.

Also a must site to check: www.mousesavers.com(This site is loaded w/ info and all the latest discount codes, offers, etc.). FYI- this is not a disney supported or sponsored site. I can tell you from experience the Disney packages (even w/ discounts) are not the cheapest route.

Sorry, I forgot to mention, we book our tickets separately then purchase hotel & park pass packages. You can certainly save money by not staying on WDW property. We prefer to stay on property for the convenience but you truly pay a premium for the convenience.

My family and I are also going to be in Disney in April and we found very good deals. I have been down there twice and I believe that you will save a lot of money if you do everything yourself and start by checking out www.biddingfortravel.com once your there check which hotels are available and how much you can expect to pay for them. For example we got the Wyndham Orlando Resort for only $34 a nite and that is including taxes that we did using priceline.com. Last year when went down we rented a compact car for only $14 a day, it was only $136 for the week. Also, last year we stayed at Wyndham Palace paying only $60 a nite and we had a blast. Everything using that the information found at www.biddingfortravel.com. Hope the information was helpful.